This may look like a car that was chopped off at the fire wall and converted to a trailer. However, this is all hand made, each piece purpose built on the spot, and made with a lot of imagination. If you are willing to put two and a half years, and $4,000+ into it (I hardly spent more than $100 at a time), you will have the satisfaction of having a trailer that perfectly matches your car.
And it's just plain fun!

I shall now take you on the journey. Each step is meant to give you an idea how to execute this creative plan, and by no means official; if you know of a better and more efficient method go for it!

P.S. Do not ask for plans or anything like that; I have none. This was straight from my head, and calibrated by eye ball.

Step 1: Count the Cost

This not only involves money.
1. Design: get it all so you know what you need to do and buy.
2. Materials needed: wood (multiple dimensions), fasteners (screws, nails, staples), glue, fiberglass, plexiglass,
insulation, hinges, lights, paint, polyurathane, weather stripping, and many other things as needed.
3. Tools: sabre, table, and circular saws, drill and bits, screw driver(s) and bits for the drill, welder, paint brushes,
staple gun and staples, fiberglass roller, scissors, saw horses, chisels, hammer, wrenches, sandpaper, C-clamps, and others.
4. Housing. You'll need a dependable place to do this. If you don't have that you can't build.
5. Help and expertise. If you have the determination, but not the ability for certain aspects, like me with welding, find a friend who knows.
6. Legalizing. Make sure it'll be street legal.

Step 2: Set Your Foundation

I made the floor as a sandwich: The 2x4's cut to their lengths and marked according to their place. They were all attached using the wood dowel method with glue. 1x6's were glued and screwed to the underside, and 1/4" plywood nailed on that from above to serve as a floor for the compartments that would form. On top 1/2" plywood was cut into sections to be the strength deck and have hatches for the compartments within the frame.
To keep the hatches from falling through attach 1x2's to the underside of the strength boards around the perimeter of the holes. Then fasten hinges and you are in like Flynn.
This was my key to the wood: glue and screw. One compliments the other, and extra strength wood glue is the way to go.
The last two pictures are the false tongue being removed.

Step 3: Set It on Wheels

Now, this is where things get important. You can't eye-ball this: setting the axle.
I used 3" "L" beams on the sides for the floor to sit in and bolt to, 1 1/2" "L" beams to tie those to a 2" square keel beam that runs the length. Another pair of three inchers come from the sides to form the triangle. The leaf spring brackets were welded to the sides, and we made doubly sure they were squared up.
Truck bed tar was brushed on the bottom and sides, and the floor structure was bolted on: 2 on each side and two on the front, and each bolt comes in from the side.

Step 4: Frame It Up

The model you see is one I did to figure out whit went where. It helps for visualizing.
In order to get the proper shape I did cardboard templates to make side ribs out of 1x6's and 2x6's. Those were joined by 2x2's forming an 8 inch internal wall that would hold two shelves at 1' and 2' heights. 1x6's were used for the roof structure, including the sunroof.
A note for the ribs: shelf brackets hold them to the floor as well as screws from the bottom.
The 2x2's are joined by notching and slots.

Step 5: Get Lit. (the Tail Lights, Not the Other)

Of course each car is different, but with my car's design I could do 1x6's for the two horizontals, and cardboard for the back, sides, and top. The trunk frame provided the foundation for building. As you see in the first picture I made a cardboard mock-up so I would know what to do.
Later on after fiberglassing it in I got a wiring kit, and I was able to use that hollow 2" steel tube to run the wires through. After much advice I got them working right!
Follow these rules for wiring: run the power wire to the light, from the light to the frame. Never let two hot wires touch: I blew fuses in my car.

Step 6: Skinning

This involves a lot of cutting to get the geometry right. After calculating the surface area I determined that 8 pieces of 2' x 4' x 1/8" plywood were needed for covering, and two boxes of finishing nails.
Unless you have a steam box you'll need to cut the sections into strips so the wood doesn't split around sharp corners and curves. On the door most of the contour cuts do not go across the whole length; the car's contour did not go across either. It makes for easier assembly.
On the front corner, rear pillar, and roof flanks I used cardboard attached with paper and glue. You can see that in the last picture.

Step 7: Fiberglassing

This is the horrible part: fiberglass, and expensive. You'll need gloves, brushes, a fiberglass roller, and clothes you don't care for anymore.
Cut the cloth as appropriate, apply a coat of resin after hardener mixing (work fast), lay the cloth, brush on another coat (new batch), and roll out the air bubbles.
Smoothing is the time intensive process. Bondo it, sand it, clean it (much dust). Use a power sander, please for the sake of the children!
You'll also notice the sheet metal on the fender. I actually lined the whole bottom perimeter of the trailer with that.

Step 8: Insides

Not to worry, I will explain the rear box in the next step.
The insulation came in the form of 1/2" sheets which were sliced on one side to match curves and glued in place.
After staining and polyurathaning all of the surfaces various colored fabrics were stapled in place to hide said insulation.
All doors latch from the inside only, except the side door. It has a stock locking camper handle.
The battery is attached to an inverter to run electric things later on (razor, chargers, tiny stove, coffee pot).
I chose to leave my compartments unstained, but this'll be your project; you do whatever.
Battery lights are the best! In the cabin are 4 pod LED's which are aimable, and on the roof flanks are small one piece lights that rotate side to side. In the trunk are solid push button lights. They just sit there.

Step 9: Extra Credit

There're always extra parts left over, like an extra 18 inches of frame. So I took left over wood and made a storage box. There was a random piece of cypress from another project that formed the step on the back.
Each of the three table pieces are on piano hinges; the center flips forward, and the two sides flip outward and are held up by wood dowels. All surfaces are stained and urathaned.
I recommend running some caulk on the seating surface of that center lid. All three of my boards have securing velcro when not up.

Step 10: Painting

You can do this one of two ways: on your own or send it out to a pro shop. I tried it myself, and decided to have a paint/body shop do it. If you do it yourself ensure all areas are covered and sealed. There's also a special thing those guys used called Fiberglass Paint. Evidently it gives it a mirror smooth finish.
After that install your windows. Go with the higher price Lexan. Yes, there will be some pain, and lower intestinal discomfort due to the cost, but it is sooo much better than acrylic. It cuts easier with a sabre saw (fine metal blade), and is less crack prone when drilling the screw holes.
I found this 3 step method the best when attaching the plexi: drill in the plastic, run a bead of adhesive on the body, place the plastic and screw it in. Use machine head screws that'll sit flat on the plastic.
Before you adhere the taillight covers put color appropriate reflective tape in there. You may see the yellow tape, and I must say that was a fluke on how I got that. The paint place also does school busses, and that is the only way to obtain yellow reflective tape, just to warn you.

Step 11: Last Little Things, Stuff, and Other Related Details

Picture 1: Handles, they assist in opening. Like I said before, the only door that locks externally is the side door, and the front and rear hatches latch from the inside and have solid handles. That way you only have one key to concern you.
Picture 2: Hatch pillars. Just random wood I found sittin' around. A small block was screwed into either side of the front door frame to hold that up, and the trunk rods sit on extra frame pieces. By the way in picture 5 you can see the trunk hinges: old recliner foot rest metal. That was a beast to disassemble, holy cats!
Picture 3: Cargo tie-downs. These are U-bolts glued to the floor through holes, and J-rods screwed in on the ceiling.
Pictures 4, 5, and 6: the gutter. Water will run down from the back window into the truck. It's a truth; accept it, embrace it. So I took my left over fiberglass cloth (hooray for extra parts) and resin and made a U shaped gutter with a shelf to hold weather stripping.
Picture 7: The vent. This was from paint job 1. If you don't wish to deal with the greenhouse effect cut a hole in your sun roof, frame as needed, seal it, and presto. Or an easier and less complex way is staple in some curtains over the windows.

Step 12: Finished

And there you have it! your very own teardrop trailer version of your car! Add your own character and flare, or not.
Like I said in the "count the cost" step Make sure your state's DMV/Police Dept is okay with it. Keep ALL receipts to prove you are the owner. What I did was scan all mine, 4 per page, and took that in. BAM! Good to go! I got a serial number and I was in like swim wear.
Check out this little secret: I'm not a carpenter, electrician, mechanic, painter, or interior designer. I'm a secretary!
So the moral of this story is determination, and God's gift can make you do anything.