If it were me, I'd use 1/8" plywood or corrugated cardboard as a core.
A few plies of heavy (non aviation, ie, boat type) cloth on the
outside, maybe one on the inside. Polyester boat resin (cheap, and
not as suseptable to heat softening). Check out the heavier cloths
here http://www.discountcomposite.com/glass.html and remember that
we're paying about $6 per yard for 9 oz glass for homebuilt airplanes.

I like the cardboard idea. Do you think it would hold up to the moisture of
the resin? Would I be able to use Bondo to smooth sharp corners?