You could try to make a subframe and staple the material to that like a sub-box. Then just screw it to a lip on the factory bed and fiberglass the whole piece. Thats just trying to help you, there may be a better idea out there.
you can go to wal mart an get the poleyster fabric dat stretchs bothways put it on from the back with resin 2 mak a base then usesome long hair fibre strand body filler an shape it den cover over wit putty coat. .i had a bed side i smashd into a sign an wen i put the new one on i kept it 2 experiment wit and it worked dam good its been riden on a buddys truck 4 a while now