Cover the styrofoam with smoothed aluminum foil, the resin will melt some foam. If the foam is the shape and size you want then the first fibreglass work is making the mould, the part made from the mould will be the right size.
Nice day to ride by Lake Michigan.
Too bad the potholes and expansion joints on Lake Shore Drive are so bad, my battery popped out of its holder, pulled off the quick disconnect and set itself afire on the rear wheel. Lithium ion no less.
My pit crew (lol, wife was pissed) brought me my track bike 4 cell and a 10mm. Above was an 8 cell. Salvaged my QD harness and finished the ride...