Unless you made it really thin, lexan would probably be about as heavy as the plastic that's used on the RS bodies, and it would almost definately be heavier than a custom-made fiberglass body (and the fiberglass body would probably be easier to make too, although not quite as strong). To make it would be a pain but not really that difficult, just make a mold of your xmod body parts and use that to make your new body. The materials needed can be bought at most lowes-type stores; mold material, mold release stuff, and fiberglass mix. You can also find it on the web (tapplastics.com is pretty good).
(I'll also note that I've never tried to make anything from a mold, I just have a basic understanding from googling it, which you should also do before buying anything
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