The best way I have used is the fiberglass the inside method. 1. Tape up the outside to keep it clean first make sure and tape good especially the bottom edge . 2. Wax the inside with release wax or just a good paste car wax works too.
3. "Epoxy resin" and glass cloth the inside only a couple layers only. 4. let set up. 5. pop out, and add a couple more layers until rigid enough to vacuum form on.