My son got a battery-powered 4 wheeler for his birthday last year, so this Halloween we decided to turn it into a fire truck so he could drive it in our town's Halloween parade. We found photos of a Seagrave fire truck online (our last name is Segrave-Daly) and used them for my design.
I simplified the side profile and cut out 3/4 inch foam insulation which I then painted to match my design. The foam sheets come in 4 x 8 foot panels, so our truck is 8 feet long; it's about 40 inches wide to match the size of the 4 wheeler, giving enough clearance to turn without bumping the wheels. I built a lightweight frame of 2 2x4's split in half which is attached to the 4 wheeler, two frame members front and back. Once the foam panels were attached to the frame, I was able to create the front, top, and back panels.
I used an old freezer base painted silver for the front bumper, printed stickers for the grille, headlights, tail lights, and gauges (we took pictures of our town's real fire trucks for the elements), painted paper plates for the hubcaps, and painted PVC for the hoses and hose hookups.
The ladders were cut from leftover foam. I also had a pair of amber USB rotating lights which went on top, and found siren videos on YouTube and then extracted the sound files to play on an old MP3 player through old battery powered speakers.
The total weight of all the components was only about 20 pounds, so the 4 wheeler was able to drive at its normal pace of 2.5 mph, even up the gentle hills we encountered along the way. When we arrived at the parade, the real firefighters loved our truck so much that they added the final touch - a stuffed Dalmatian puppy to sit on the back.
Total cost of the things I had to buy for our Homemade Fire Truck for Fire Fighter Costume was about $50; with design, painting, and construction. I completed the project in 15-20 hours.