i just made an airbox out of perspex the other day.
i lined it inside with some silver faced heat resistant foam. but it didnt look very subtle under the clear perspex. so i thought i might paint it black on the inside.
in the process of testing paint on a scrap piece, i decided to see how it deals with heat.
i was surprised to find the perspex is pretty damn heat resistant. i placed a piece on my palm, and aimed a jet flame at it till it started to melt, but i didnt get any heat on my palm. i'm impressed !!!
so there you have it. perspex. easy to work with. easy to shape. cheap, and pretty effective.
obviously not painted yet. and i will probably only insulate the side, not top.