For your molds, if you fill the back with spray foam it will stiffen them up enough to make accurate copies, with heat forming, the resin may produce enough heat to deform the molds so they'll noty come out exactly right.
to hold the material in place, use "T" headed straight pins, most sewing shops carry them, built a front end for a fiat 850 spider years ago, raided my Grammas sewing cabinet, and those pins worked so well I keep a pack around for any fiberglass work. Lay out the cloth trimming it for best lay, pin it down, brush on some resin then pull the pins when it starts to gell. I use a small hooked pick so I can pull them without disturbing the resin.
My molds are generally for one off parts, either for a solitary project or to make a production mold from, so low cost is normally the main concern. what I normally use is spray foam or plywood sometimes styrofoam depending on size to produce a base to start from, then apply a thick coat of a fine plaster that will sand to a smooth finish. sculpt it then sand and finish with a very smooth coat of paint, normally use household latex paint. then pull a mold off that. for one off projects I do the same but allow for the thickness of the part itself and use it as a plug, requires carefull finishing of the part as you're working on the finished side.
also, Epoxy resin is much stronger than regular polyester resin, so for a part that is subjected to stress, it may be the better way to go.
All resins are sunlight sensitive and break down with exposure to UV, (reason why gell coat chalks) most clear fiberglass has a UV blocker of some sort, so if you want to see the weave, maybe a 'candy' paint?
Check an auto paint supplier, see what they use to custom blend poly or epoxy paints.
ken.
