Fiberglassing Outside of Car

I came up with a project idea the other day. If anyone is familiar with the 98 Tahoes (older body style), in the cargo area there are two panels which extend out from the wheel well humps to the end of the cargo doors. I wanted to create "replacement" panels for these and was thinking that rather than laying the fiberglass down in the car, is it possible to create a plaster of paris mold of these first, and then fiberglass them outside the car? Also, what kind of material other than plaster of paris can I use to create a mold to apply fiberglass to? Is there any types of foams I can use that won't melt from being glassed?

You could just take them out, mask them off and lay glass right on top of em. A little trimming and it should fit just right.

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Nik
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