I would just use something rigid and very flat like sanded wood, glass, acrylic, steel, aluminum, flat tile, or stone (granite or travertine). Anything that you can make a 90 degree right angle. Then put a layer of parchment paper, wax paper, zip lock bag or saran wrap in between the cast and tank. Anything to prevent the cast from adhering to the tank and that won't adhere to the epoxy, or if it does, anything that is easily removed. If your unsure, test it first. Fill the chip with the epoxy, then clamp the cast, remove any excess epoxy with mineral spirits or acetone and let it dry for 24 hours. If you don't have a clamp you could use something heavy to hold it in place or you can buy clamps for cheap at Harbor Freight.
After it's dry, if it's not aesthetically pleasing and you want to clean up the appearance you can "carefully" sand it smooth. Heavily tape off the glass so you don't accidentally blemish it. Start at a low grit working your way up to a very fine wet sand. You could even go a step further and buff it with a rotary tool and compound and then buff it with a polish. Do a search on fixing fiberglass boats. Similar process. It's in the back so it may not be worth finishing it or if your cast is really good you may not notice it at all.
I'm not certain what kind of strength the repair will achieve, but it's better than nothing. Some epoxies have a holding strength of over 4000 psi. More than enough. The key is getting a good adhesion. The chip appears to be rough so you'll probably be fine.
Good luck, let us know how it turns out.
