Thank you everyone, as this Instructable was one of the 1st Place winners of the UP! Contest. I appreciate the support!

This Instructable covers the building of the mini Cadillac body shell that I attached to the scooter I ride with The Krewe of the Rolling Elvi. We are about 120 guys who dress like Elvis and ride small, motorized scooters in a couple of New Orleans Mardi Gras parades, and for other events throughout the year, such as The Running of the Bulls. Each rider is encouraged to pimp-out their scooter. I decided that THIS Elvis would like to ride a mini version of his epic pink Cadillac.
I tried to take a good number of photos to illustrate the process that I undertook. I realize now that I may have missed a couple of opportune moments to capture a technique or process which may have been helpful...sorry. If you happen to have any questions about a particular step, feel free to message your questions to me and I will try my best to answer them. Understand that most of what I did to make this was very new to me. I simply thought out the processes before beginning and if I had any questions, I called a few hobby shops or hardware stores in town for a little advice. I am glad that it came out pretty close to what I had expected. I hope you enjoy what I did here, and that perhaps I have inspired you to do a build of your own. This technique would work on an endless number of items - not just mini car bodies.

Step 1: Carving the Car Shape

I started out by looking online for views of a 1955 Cadillac Fleetwood car (The epic model) with which to model my car after. After finding a couple of decent images I printed them out and sketched them onto a large sheet of paper (frontal view and side profile) - the size that I felt would work best for attaching to the scooter I was supplied.   From there I transferred the outlines/details to the foam board that you see in the photos. I used an Exacto knife, a rough hand-held rasp and a Japanese-style saw to shape the foam board. The foam board was made by Dow (it was probably meant for insulation) and was 2 inches x 2ft x 8ft in size. This project took two boards to complete (each board was about $15).   I used Elmers white glue to join them and found it worked fairly well (I have to admit I love Elmers glue). When using the glue it is best to use it in smaller amounts…. dries quicker and more glue is never necessary. I used long clamps to hold the boards while drying.
Frontal view of the front of my soon-to-be Cadillac model. You can see that I prefer the Gold Toe brand of white sport socks – very durable.
     Just doing a little general shaping at this time. It is important to be conscious to NOT make any undercuts in your creation as the applied mold will need to pull as straight off of your final shape as possible. So instead of having undercuts, it is preferred to have sloping sides of items such as the bumper and headlamp details.
     For additional details, you should read the notes that are attached to select photos.

Step 2: Add Compound to Smooth

These seven photos illustrate the application of the Sheetrock All-Purpose Joint Compound. I chose to use this material in order to gain a final smooth surface on the car body. I knew that the compound would stick to the foam, and I also knew that with continuous applications and sandings I could gain the desired surface texture and details that I was looking to achieve. This step took a long time with all the many applications between sandings.
Once again, check the images for more details...

Step 3: Adding the Logo Emblems

Here are two photos showing how I made the Cadillac Logos on the front and back sections. I found the logo online, and simply traced the logo onto a piece of card stock, and then made a stencil by cutting it out with an X-acto knife. I then positioned it where it was to go, and dabbed on a thin layer of sheetrock compound through the stencil. I had to be careful when removing the stencil so as not to disturb the logo. It is best to try and hinge the card stock in position so when the template is lifted it will not move much side to side. In this way the removal is relatively clean.
After the compound dried, I had to lightly sand the logos as my compound application was not very uniform - and besides, I only wanted a thin logo to indicate positioning for painting purposes. Looking at the rear section, I notice that I did put the logo on before I was finished smoothing out the trunk area of the car in the first place... oh well, I fixed it afterwards. There was not a great deal more I had to do after this point.

Step 4: Preparing for the Mold

Here you can see the front of the Cadillac, with the Logo Emblem in place. I have begun to brush on the petroleum jelly as a mold release on the passenger side of the car. You can see where the petroleum jelly makes it a shade darker and shinier. This application will help to prevent the plaster of paris mold panels from sticking to the original model. It is good to be somewhat generous when applying the mold release agent.

Step 5: Making the Mold

OK, this is one of those times when I realize that I did not take enough photos of the process. For instance I would show that in order to make a mold, one must think about how one must understand that when all the mold material has been applied to the original model, that one has to know how one is to remove the panels from the original very cleanly and with as little trouble.
     You want to assure yourself that you will NOT encase your original in a shell that cannot be removed without possibly damaging the original. You also want to make MULTIPLE panels that can INTER-LOCK together when removed, which can indicate exactly where those panels are to be positioned to assure an accurate reproduction of the original.
     STEP 1. Determine Where The Individual Panels Will Be Located. I looked at my original and decided that I would need a Front, Top and Two Side panels. Each panel's edge would be where it transitioned to the next plane. For instance, the front section panel's plane edges would be along the top edge of the headlights and along the upper curve of the hood, along the sides where the left and right side planes meet up with the bumpers.
     STEP 2. Mix Up Your Plaster Of Paris (POP) (wear gloves) Mix enough so that when you add the POP to the water, that there would be enough to satisfy making a single mold panel (one face/plane) that is around 1/2 to 1 inch in thickness (although if you add reenforcing fiber to the mixture, you can possibly make a thinner mold.
     STEP 3. Goop the Plaster of Paris On Your First Panel: The panel you are working on should be as horizontal as possible. You want to work kind of fast as the POP starts to harden somewhat fast. Do your best to eliminate any bubbles in the mixture. When you have the first panel covered I would encourage making the edges of the panel (where they will meet the next panel) a little thicker so that you can carve FLAT the joining walls between the panels, and also carve some KEYS to aide in interlocking the finished panels together. Cut these keys while the plaster is still somewhat soft. I cut V-shaped keys every 6 inches or so along all sides that will interlock with another. The edges that will not be meeting another panel, you will want to have them built thicker as well, however you also want the plaster to overlap the edge - but not too much - just enough so that when a cast is made from the mold, that you can tell where the edge of the car body is. After all, remember that you will have to trim the edges of the final fiberglass cast.
     STEP 4. After EACH plaster panel of the mold is somewhat dry, and after the keys have been cut, take a sharp tool and make sure the keys/walls are smooth. You MUST add some mold release to the POP WALLS of the panel you just made so the next panel will not stick to it. You may want to check the release on the original carving - perhaps add some more.
     REPEAT STEPS 2 & 3 for each side/plane of the car model. You will have to support the model AND the applied plaster panels as you turn them in unison to apply the plaster to the next panel. After you apply each additional panel, I've found it helpful to shave the joints so the division line (break) is clearly visible.
NOW LET ALL PANELS DRY WELL... It should be almost a week. Keep stored in a climate-controlled area to aide in getting them to dry completely. I believe it is also a good idea to take a sharpie and number the panels in the order that you applied them. In this way, if you put them together in the same order, the pieces are certain to fit.

Step 6: Disassemble Mold

After the plaster of paris has dried fully (It should take days, and you'll find that it won't feel cold to the touch), carefully take a flat-head screwdriver and slowly pry apart the panels in reverse order of application. Take each panel and lay it down where the entire panel is supported fully. This will assure that the panel won't possibly warp...a good precaution.

Step 7: Laying Fiberglass in Mold - Front Section

Once you get the panels off your original, it is time to prepare the mold panels for laying in the fiberglass matting. Remove any plaster crumbs from the mold panels. Take the first two panels that you made and turn them upside down and align them together as they should be. You must support them very well along their entire length. In order to assure that the first two panels are dead-on accurately alligned, you take the OTHER panels, and take turns holding them in place to see how well they would line up if they were all assembled.
     You must apply mold release on the plaster mold panels to help the fiberglass release from the plaster. I used petroleum jelly again for that purpose. I brushed it all over. There may be a more appropriate mold release product, but I just used what I had.
     Prepare yourself by having a wide variety of fiberglass matting cloth cut to various shapes & sizes that may benefit the areas you are to apply them to in your mold. For instance: in the conical "bullet" bumpers I had cut some matting in "V" shapes, and small patches, to facilitate getting the matting as far in as possible. Now be aware that there may be parts of the car body which have thin elements that will need reenforcing. The Cadillac's bumper's elements are rather thin & fragile and I felt the need to add strength. I used some wooden dowels to help support the "bullet" bumpers. I had some Popsicle sticks for pushing the matting into tight areas. I also used some plywood angles to help strengthen the bumper to the hood. When adding wood molding to your cast, simply slather these wooden pieces it is advised that you press them onto the previously applied matting/cloth, and then seal it in with a coating of resin and more cloth.
     This first photo is of the car's hood and the front grill panel in place – well supported, and upside down of course. I have begun laying fiberglass in place. Use a chip brush to apply a layer of resin in the nooks and crannies. I wish this photo was larger for you.
      On the back section of the car I put the quarter-round along all lower edges of the final fiberglass cast. it gives me something to hold onto when moving them around.
      All in all, I guess I made the shell about 1/8" thick all around. Areas I thought would need additional strength I made thicker - like edges that would be handled or bumped into. I also added the wood molding for that purpose. After you de-mold the cast you can still add fiberglass to it so don't be too worried that you didn't put enough.

Step 8: Laying Fiberglass in Mold - Rear Section

Use the same procedures for the rear section that you used for the front section. You will notice in the first image that I have an industrial lamp close to the applied fiberglass because it was a very cold night, and the heat from the lamp helped the fiberglass to cure. This was probably not really necessary, but I thought it couldn't hurt. Be careful, as fiberglass is flammable. I happened to be outside on my back patio working on this very cold night. I wouldn't have done this if I was inside, and I wasn't inside because of the fumes.
When you remove the casts from the mold sections, you will find that the edges are very rough and areas of the grill and headlamps are filled in. You will need to clean those edges. I used a pneumatic tool & cutting disk hooked up to my air compressor. It cut through it like butter... be careful doing this. After trimming the edges I did find that I had to add more. Easy to do. I like fiberglass work.

Step 9: Mounting to Scooter

Front Support:
It is made of perforated galvanized angle iron, 1/2" x 1/8" solid angle iron, flat steel stock, pipe clamps and a few zip ties to help it in position. It was necessary to mount the support structure to the small, curved frame of the scooter. Luckily it had a curve in it, which actually provided a stable place for sandwiching the two sides of the support structure. I needed to make sure that whatever structure I put on the frame would not interfere with the normal operation of the scooter.
Rear Support:
The rear support I made out of flat steel bar and part of a purchased bicycle book rack. The book rack was disassembled and I only used the support rod – I threw away the plastic shelf.   The seat support was a little too thin in diameter, so you can see the red piece of rubber gasket material I had to use to help make it a firmer, stronger structure.  I used a propane torch to heat and then bend the steel bar where I needed to. I drilled, and then bolted, the two pieces together – you can see the blue locktite-coated bolts.
     I took both body sections and laid them on top of the supports I constructed to see how well everything lined up. I did have to put a piece of small angle-iron across the rear support structure at a right angle. This helped to raise the front end of the rear section, and to reduce any wobbling.
     You can see the the last two photos how I fiberglassed into both the front and rear section a small sheet of plywood to provide a firmer place to bolt the sections to their respective supports on the scooter.

Step 10: Headlamp Installation

The first two photos illustrate how I used a hot glue gun to install some plywood discs to hold small, button-cell, LED lamps for the first parade. I soon determined that the cheap little LED lights I bought were not going to be good enough. I ended up buying two high-intensity, LED bicycle flashlights for headlamps. The remaining photos show how I drilled out holes through the plywood which perfectly fit the flashlights. I also used J-B Kwik 2-part epoxy to mount wood blocks to the underside of the hood, behind the headlight holes. I used these wood blocks to mount key-type hose clamps, so I could hold the flashlights firmly, yet easily remove them when I want to change the batteries. I wrapped a little strip of rubber around the flashlight, where it lines up with the key-type clamps. The headlamp mounts work beautifully.

Step 11: Finishing Touches

At this point all of the structural aspects of the body panels are finished. It is now time to position the panels exactly where you want them, and as someone holds the body firmly in place, drill holes for your bolts up from the bottom through the body panels. As you drill a hole... put a bolt in place with a wing nut... not too tight. This should assure that all the bolts will indeed line up once you are finished drilling. (I recommend this method because I HATE when I mark all the locations for holes to drill, and find after drilling that something had moved and I am off for some of the bolts. Grrrr) By the way, I use wing nuts to secure the body panels to my scooter because I need to be able to easily disassemble my scooter/car to transport everything to the parades, or other events.

Unbolt the body panels and add the details to make the car unique...

Trim the edges of the body panels and sand them smooth.
You should wash the top of the panels with a hose and a good cleanser to remove any mold release residue. It wouldn't hurt to wash it more than once.
Chances are pretty good that there are some imperfections in your cast. You may want to Bondo the holes/pitts that could show up on the surface. Sand those areas after it cures. (I have to admit that I skipped this step as I was in a hurry to get my car/scooter into it's first parade. I hope to one day go back and fine-tune the finish. It still makes an impact though)
You certainly have a couple of options available to you now... bring it to a car body shop to have them paint your car with auto-quality paint, or you can paint it yourself. Like the professionals, you will want to prime it first, then paint.
I also went to a sign shop to have them reproduce images of Elvis and Ann Margaret (considered the "female Elvis" in her day) as a sticker to apply to the windshield. I don't know why, but I decided not to use Priscilla.
I used plumber's aluminum tape as chrome for the bumper, around the headlights and various trim.