I did it off a Valley Pintail seat. Instead of glass, make your negative mold with plaster of paris. I covered the seat with plastic wrap. Use several pieces instead of one to form the pockets with out air pockets. Fill it with plaster of paris. Once removed, I burned off any adheared pices of plastic with a propane tourch. Sand it and fill any voids with wall joint compound. Sand it,then coat it with epoxy.

Wax it and glass it. The surprising thing is how much glass it takes to get a stiff substrate. I use 1 layer of 6 oz. A layer or two of scraps - 6 oz. Then a layer of roving. Then about 3 more layers of scraps of 6 oz. I add a little copy toner into the mix to make it black. I use a fast epoxy and do this in the sun. In about two hours I can remove it from the mold while it has a lot of flexability. Once it hardens like a rock, it can be stubborn to get off the mold.

For a release agent, I use a layer of Auto wax and buff it. Then a thick layer of Auto Wax that I don't buff. You could buy real mold release, but they're kink of expensive. You can also use wax paper laid on the mold in pieces to comform to the shape. Some people use melted parafin painted on. I tried this but found it gets lumpy.

Once it's done you can smooth it out with epoxy putty.