[lunarhighway]

i assume the bellypan itself isn't structural and therefor if you want to save weight, just use thinner aluminum... i've used very thin aluminum on mine and it holds up perfectly. or perhaps some kind of fabric like on ww1 aircraft could work(although that might come out heavier than the thinest aluminum) corrugated plastic is as always a good idea too.

i'm not sure what the requirements are for what your building, will it race , mingle with traffic or whatnot.

weight reduction will give you most gains in situations where you have to stop and accelerate a lot (city traffic) while aerodynamics will pay of more on the highway... i presonally think that the added weight of a lightlweight undertray, will be more than canceled out by the aero gains.

a more difficult engineering exersize might be to make (parts) of the undertray a structural element... like the stressed skin on most WW2 fighter planes allowed the use of thinner internal structures (or for those to be perforated) wich imho would see bigger weight savings while allowing perfect aero. brace wires might also be an interesting idea to make certain structures more rigid while allowing the heavy structures to be lighter.

on the other hand there's something to be said for over engineering as well when it comes to safety etc... so it all depends where the priorities are