The best way to glue plastic is to solvent weld it, like PVC pipe. this is not something you want to do with the tank in use. you would have to find out what kind of plastic it is and find the proper solvent to use. Your best bet is to get a replacement from a LFS, or notch out the existing brace to allow a 3" wide peice of glass siliconed in place of the failed brace.
I would add, that when using an epoxy or a solvent type weld, sandwich the brace between some other, or the same material. That will give the best strength for what you are trying to do.
i just brough a top frame from the local fish store for fifteen bucks...replaces it an have had no problems...i was tired of messing with gluing and **** so my opinion it will save time to just replace
