By Don Kinyon (extracted from a PVAS forum thread)
After water changes were done last week, I was feeding the fish and heard a load "pop" from the 55 gallon tank at the far end of the room. The top brace had broken away from the plastic rim - lucky the whole thing didn't come apart! The long portions of the glass were bowed badly, so I drained out about half the water just to keep the thing in one piece for the night.
Next day I took an old piece of plexiglass, propane torch, circular saw, some PVC cement and RTV silicone seal and went to work. The plastic was cut into two pieces about 8 inches by 9 inches, then heated enough to bend a 1 inch 90° angle on the longer end (leaving it approx. 8x8). The surfaces to be glued were sanded to rough them up. I put a bead of silicone on each side of the tank rim to hold the brace in place, glued the mating surfaces of the new plastic and clamped it into place.
It's not something you'd do with the display tank in your living room, but if it's a tank that just needs to hold water and fish dependably, this works great. I've got a couple home made tanks like this that have been in service for over 20 years, so I'm thinking it works.