I haven't really decided yet, but maybe something like
Vintex vinyl/nylon fabric with
reflectix radiant insulation on the inside. The Vintex stuff, and probably any vinyl coated cloth, is cool because the seams can just be glued together. No hassle w/ sewing machines.
I was thinking originally about having it permanently attached at the bed and the shell, but it might be hard to manage when putting the top up/down if there's more than one layer (like with insulation). Could attach it at the base of the shell and then fold it down and clip to the bed when the top's up.
Do you have any recommendations on materials to look at?