If you're going to use the lost foam process, make sure that you've got enough clearence so that when the FG is put on, it actually fits.
Also, use epoxy resin. Polyester resin will eat your polystyrene
The last airbox I built: First, I taped the backs of the reed blocks. Then I got some fine alloy mesh from Mitre Ten, shaped that in the bike to the shape I wanted, and taped the inside with duct tape (the proper cloth stuff-think the brand I used was bear), and capscrewed on the two plates I had prepared over the reed blocks.
Then I fibreglassed the inside till I was happy, and when it set,
I used a dremel to get at the capscrews-put a little circle of tape on the back of them so you can unscrew them, and took them out, so I could remove the box.
I discovered that the Duct tape has an oily layer-the one that stops it sticking to itself- and it all peeled off in one hit
Then I fibreglassed the flanges on at the top, cleaned it all up, and bolted it in.
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The Chopster