Most local hobby stores that deal with RC cars will carry sheets of lexan/polycarbonate. It's used to fix them car bodies, and also to make windsheinds for planes! You can get stiff stuff, as well as stuff that's fairly flexible. I go tsome recently for exactly the same thing, but it was a 1/4 scale cub..

Make a paper cut-out first, before commiting to the lexan. Start off with a triangle with a pretty generously curved bottom edge. cut 'arm holes' in the sides to fit around the wing leading edge protrtusions. It'll be a bit of trial and error, but patients will pay off.

The other thing to do is contact a kit manufacturer. They're all pretty close, that is, they all copied the same plane, with about the same dimensions... Pretty much all of them will sell you the extra bits. Fiberglass specialities will sell you a fiberglass cowl...