Well, in that case, thinking it was just a pan, you might want to consider using the same product the fiberglass unit is made of, which is fiberglass.
You can use this product bought at any auto parts establishment that has fiberglass repair kits.
This product is tuff as nails and could very easily repair this unit by completely creating a new smooth surface at bottom of unit. I have in my years in the profession of plumbing repaired holes in fiberglass and either covering up units with holes in them with add-on items such as soap dishes or other items.
This would be my next best suggestion. That fiberglass repair kits are actually tougher than the product you are working on.