1: The foam is available at Lowes or Home Depot. It comes in different thickness so pick the one that will suit your project best. Cut the ruff shape with a utility knife the start shaping it with 80 grit sand paper on a sanding block.
2: You have to use epoxy to coat the foam as fiberglass resin
will immediately eat the foam! I found my epoxy (medium set)
at Boater World made by MAS Epoxy. You will also need fiberglass cloth which is much easy to use than fiberglass mat.This is also available from the marine store or on ebay.
3:Mix the epoxy as indicated (try not to waste it, its expensive) and spread it on the foam with an acid brush. When its all coated place your glass cloth over it and apply more epoxy. Once its dry block sand it lightly. At this point
I use body filler and sand reapplying filler till its smooth. I then prime it and apply a guide coat and sand with 180 grit
and repeat this again until there are no low spots and then finish with 320 grit. Its then ready for you paint finish.