Hi BajaPatient. Thanks for the input. I had thought about using the vacuum technique and, I'm sure, if I had access to a vacuum pump, I would have tried it already. Certainly seems like the best option for small-scale or high-value production but seems a bit steep for a one-off. The alternative technique sounds interesting. How many car sponges do you think are needed??
TBH, this particular vehicle is more experimental than anything else. It's only about 22" long with a 1/14 scale chassis. Should be good for about 100mph but I'm using several aspects of this project to learn about construction and control techniques for a faster vehicle. Worth noting that the current world record is over 170mph.