I know how to use fibreglass, for one of my old cars I made a plug and then created a one off bumper from scratch, an airbox would be easy.
Make it up in cardboard, over that with tinfoil, cover that with floor wax / release agent, then apply strips of glass mat that have been dipped in the resin.
Leave for a day or so and wa-la.
Hardest part is getting the mat and resin cheap. You can either get woven mat (carbon fibre type idea) or chopped strand.
Remember to wear a mask when you playing with it, and especially when you cutting it with the angle grinder
http://www.cardomain.com/ride/27008/4