Homebrew PCBs

Regarding fiberglass, how about this crazy idea:

Find a sheet of polystyrene that has the desired inside dimensions of the tank.
Double-bag it in a polyethylene bag, one that holds water.
Fiberglass the outside of this, leaving one edge open.
Open the bag, fill in a small amount of solvent (acetone, petrol,
paint thinner) which will dissolve the styrofoam.
Pull out the inner bag with the styrofoam mess.
Pull out the outer bag away from the fiberglass.

Crazy, but might just work. I'm sure there are more practical options
like making a collapsing mold or something.

Does epoxy resin even hold up long term?

ST