In our Cub Scout pack, the adults also create cars (to keep us from building our kid's cars LOL), but the adults have no rules except the car have to weigh at least 9oz and contains some wood. I printed with pine wood filament from ProtoPasta. (Photo is black ABS version).
The car doesn't weigh enough without weights on the outside, so I added a compartment inside for weights and a hatch on the bottom. Compartment extends above the rear axle. Total weight of this version in wood filament with 100% infill is 4 ounces.
The compartment was designed to hold more than enough of the long round and half-round steel weights found at your local hobby store to get your car over 9 oz. I suggest using metal putty or half round weights over the rear wheels.
Round weights also fit in the round jet port in the rear.
I glued the bottom compartment cover in place when printed in wood filament.