[DonRotolo]

I would experiment with Balsa wood, since it can handle the curing temperatures, is stable and fairly light. Aircraft use it, as well as honeycomb aluminum - got a small airport nearby (of course you do!) - call the flight school and ask them what airplanes use as a core for prepreg, the'yy usually be happy to spend 15 minuts on the phone.

There are some high-temperature foams, but the weight savings (perhaps a few ounces) versus the costs (you don't want to know) might not be a good deal.