This year, my son decided to build a "Dukes of Hazard" inspired HMMWV (Humvee). These steps can be used to make any number of vehicles.
Disclaimer: The use of power tools in tales certain inherent risks. Proper protective equipment should always be used and tools should be properly maintained. Children should be closely monitored and aided if they are to use any power tools. I am not responsible for any injury you may sustain.
Step 1: Materials, supplies, and tools
1 pinewood derby kit
additional wood (we used balsa, so as to minimize additional weight)
wood filler (sandable)
sand paper (we used 120 and 220 grit dry, and 320 and 600 grit wet)
masking or painters tape
saw (a band saw works great)
power sander (optional)
Dremel tool (optional, but very helpful).
Step 2: Pick a design
As I said earlier, my son wanted to build a HMMWV. So, we did an image search on Google for an M-998. We found these illustrations from the military training manual for the HMMWV. We decided that a "fastback" design would work best for my son's plans, so I added a couple of lines to the design.
We then printed out our design with a length of 6.5 inches, since that size best fit our block of wood.
Note: Make sure that your design meets the specifications in the rules. You may also want to check to see if there are other restrictions imposed by your pack or counsel.