You've got your flight suit, Proton Pack, & Neutrino Wand- what's next? Ghost Trap!
Reference and Build Materials

I used pics of one of the original movie props and the excellent plans on the GBfans website- Stefan Otto's Prop Plans for dimensions. I was going for a prop that would be light and would hold together while walking around a Con so I didn't include working doors or a removable inner box. I'm sure those features are doable but they weren't a must-have for this first version.

Materials:

Hobby knife, xacto or equivalent
extra blades (dull blades are nothing but trouble)
foam core board (I used solid black but it isn't mandatory) scrap board is fine if it is flat.
small amount of cardstock
hot glue gun
small section of black plumbing pipe for handle. 3/4" x 4 3/4"
a few tongue depressors/popsicle sticks

illuminated toggle switch, lamp, male air line or coax cable connector

black spray paint
silver spray paint
yellow spray paint
painters tape

optional: 3D printed details like knobs dials. If you don't have access to a 3D printer you can use polymer clay and sort lengths of wooden dowel or whatever you are comfortable working with for the little detail parts.

holster material: flexible 3-ring binder, zip ties

Step 1: 3D Printed Parts

You don't have to own a 3D printer for this prop to look good but it doesn't hurt. Libraries, maker spaces, and even certain stores have printers that you might be able to use if you want to give it a try. The 3D model sharing site Thingiverse.com, has a great model of the whole trap as well as PKE meters and other GB gear available for free. I own a really small printer and just printed the bits that are visually unique or easier to work with when they are made from hard plastic rather than foam or board. Not all models are going to print correctly on all printers, the little hose connector in the back of the trap never printed for me.
Fimo or Sculpy clay would fill in if you want to make the details by hand.

illustration credit; image 1: Thingiverse, image 2: Eric Quakenbush

Step 2: Building the Outer Box 1of2: Battery

There are two main parts to the ghost trap: an inner box that captures and contains the captured spirit and the outer box that powers the trap and allows for safe handling. Out of the two props that I built I had more luck starting with the outer box first.

Starting with the battery at the rear of the outer box gives you a stable place to work outward from. the handle attaches to the battery as well as the side and bottom panels that hold the inner box. The back end of the battery is shaped with angled corners, you can get fancy with the cutting and bevel the edges so everything makes a nice joint or go rough and leave a gap. A bit of tape over any gaps before painting will usually suffice. You can build the top, bottom, and back of the battery and just snug it up the back panel of the outer box.

I used an illuminated toggle switch from radio shack, I didn't hook up power but that is an option. there is an iluminated indicator behind the switch that I didn't catch on my build. finish up with an air hose connector in brass or silver or a tv cable connector connector. these should cost less than $10 but the really make the prop look like a real device. Tape off the switches and other gear before the painting stage.

illustration credit; image 1,2: Eric Quakenbush, image 3: Stefan Otto

Step 3: Building the Outer Box 2 of 2: Handle, Sides & Bottom

I used a small section of black plumbing pipe for my handle. 3/4" x 4 3/4" I intended to use a 3D printed handle but I couldn't get a successful print. I did get a portion of the vertical post and I built that out with layers of foam core. If I were doing this again I would cut the back panel to include the front of the vertical post (as it appears in the line drawing) and then build the post up in layers up toward the back. You might want to slip some thin wood or a couple of tongue depressors/popsicle sticks in the layers to give yourself some support.

The sides panels are going to be mostly covered by the inner box and the outer detail plates so if you have scrap foam board or, in my case, previously used foam board go ahead and get it into the game. The bottom can have cut outs for the wheels and axles if you feel comfortable cutting small windows out of the foam core. You could just glue half-wheels on and indicate the axles with a strip of card stock, it knocks the height down a bit.
3D printing is the only way to go with the 2-part sliding mount that goes between the wheels. I found the model in a set of parts for the Proton Pack. You can buy a real metal one but I'm guessing it is pricey. If you are going the non-3D part route I would just mount an 'S hook' toward the back end of the bottom panel so you can wear it on a belt. These hooks come in a range from vinyl to steel and are pretty cheap. I can see the merits of doing both hook and mount so you have mounting and display options.

Attach the sides and bottom panels to the battery. There should be about 7 inches inside the outer box for the the inner box to fit into. The bottom angle cut on the sides might show a little gap by the battery, that is OK.

illustration credit: image 1: Stefan Otto

Step 4: Side Panels

The side panels attach to the walls of the outer box and have most of the geegaws. I just cut my panels from foam core and added a small rectangle of card stock to each panel for the knobs to live on. The left panel is ready for silver paint once you attach the card stock, the right panel can be painted before or after the addition of the heat sink as it is silver too. Don't forget to hit the edges with paint too.
If you don't have the 3D printed parts you can fake a heat sink with lots of strips of foam core or card stock, i used this technique for the little grill next to the dial. The dial, resistor (that is mounted on the heat sink) and red tubes are pretty straight forward.

Bonus realism if you add screw/bolt heads to the corners.I had trouble printing full bolts but the heads print no problem.

Step 5: Inner Box

the Inner box is pretty simple with the exception of the doodad on the front/top edge. there are two examples in the photos for solving the power meter thing, the first one I did was just a couple of bits of foam core to indicate the shape. the 3D printed piece really captures the mass of angles. I added a strip of shiny green and red foil from a lego bag as a power indicator- this could be upgraded to real lights or just have a bit of red card stock in there to add some visual interest.the knobs in front sit on a bit of card stock painted silver.

The doors can be masked off with painters tape after you cut them to size. <y yellow spray paint was really old and gave me a really gnarly aged/abused effect for free. take the tape off relatively quickly and carefully to avoid it sticking to the paper layer of the foam core too much. You can run a thin line of tape down the center to give it a visible split or go in with a sharpie after painting.I scored my lid and it was a bit too subtle to see in most settings.

optional step: Sealing
You can seal the foam core with elmers glue (PVC glue) I use my finger but a disposable brush and a bowl or cup of watered down glue can work also. Sealing will give the surface a bit of shine that reads more like metal and make the foam core a little more durable.

Step 6: Optional Steps: Stickers & Belt Holster

Stickers
I went to the con without stickers but I had leftovers from a proton pack build so I've added a couple since the show. I found a set at the GBfans website that I printed on sticker sheets at a copy store (most copy shops have both matte and glossy sticker stock on hand and copiers/printers that can handle the thicker material.) the first image here could work also, just print at 50%.
Holster
The Mattel collectible trap comes with a belt holster and thought I could make something similar out of cheap materials. I used a black, flexible plastic binder that I picked up for less than $2 and cut out a rough rectangle shape that is about the same size as the trap plus a bit to fold over a web belt. I added the second half of the mount and fastened everything with zip ties. The extra strap is just a strip of binder plastic and sticky Velcro.
I saw one of the Mattel traps fly off its holster at the show- if the $300 collectible can do that then I can't offer any guarantees on this $10-15 prop staying put. I ended up using a spare strap to hold my trap on my belt after my prop hit the floor. I tried a quick release cell phone belt connector but the trap tore free (I didn't have the holster yet.)
Have fun busting ghosts. If anyone tells 'who are you going to call?' Remember to yell 'He-Man!' to establish your movie knowledge cred.
more crafts and props at www.actiondashpacked.blogspot.com