Keep in mind that any "polystyrene" foam will "melt" under the resin typically used in fiberglass work. This is because the typical resin is styrene based and reacts with polystyrene. If you use epoxy resin, it should work with polystyrene foam - i think. Or, you could use urethane foam board (usually the yellowish foil-faced stuff) with typical styrene resin.
edit: I apparently didn't drink enough coffee this morning... *asleep*
The Man is talking "polyurethane" foam, NOT polystyrene. Polyurethane foam is excellent for making plugs for fiberglass parts, etc. It will not melt when glassed w/fiberglass resin. Check some of the home supply stores or where a/c-heating units are sold. Here, in east coast S.C. a 1"X4'X8' sheet is under $20.00. You could keep an eye out for local schools re-roofing old schools. I lucked up on 180 sheets, 3" thick just for hauling it off & the contractor didn't have to pay landfill fees. Foam on. Rick
Urethane foam also comes in different weights, depending on what you're doing. The home store stuff is light - maybe 2#'s per square foot. I have stuff here up to 30#'s a square foot.. not cheap, though.
Guys, what I'm talking about is liquid. You mix two parts to activate it, then it expands.
The foam is rigid when it hardens. It can be sanded and shaped to make molds for fiberglass.
When I was building S.C.C.A race car bodies we would use it to make a buck, then make the part in glass.
then if we were happy with the new fiberglass part we would make a mold of that part so we could mass produce.
Poly urethane foam comes in 1/2 lb 2 lb 4 lb 6 and 8 lb densities .
The denser the foam the less cubic feet of foam it produces.
Thus the less dense it is, it has more and larger bubbles( air pockets) in it and produces more cubic feet.
In race cars section of the body is filled with this foam because is also impact foam and very light as well.
I can also be used in boats because it floats.
I figured someone here has used it for fiberglass here and had a supplier.
My local supplier in Kansas City has closed his business.
Now I am paying 50.00 for a gallon that produces 6 cubic feet at the most.
Available at Home Depot as a material for setting fence posts. For under $15 a bag, you get both part A and B, that when combined begins to foam in 10 seconds. Tried to load pictures but not successful. See my build journal #309 for more info.
I just use the DIY spray can stuff, not very dense but good enough for bucks and padding. I cover with gaffer tape to make things like dash under sections, quite strong when covered and then covered with fabric or PVC etc. to make a buck I usually cover with brown packing tape and then silicon but then I am crap at fiber glass work. *pbpbpbp!*