My plan would be to build a wood frame out of 1x1s, held together with screws and flat L brackets. I would attach a plexiglass center brace to it. This DIY frame would then be placed around the existing plastic frame and super glued to it.
This is for a 75 gallon tank, with a plastic frame that has cracks at all 4 corners, missing a couple sections along the back and side, and has a crack in along the front of the center brace.
Yes, I have looked into buying a replacement frame, but have been told by many sources they are specific to the brand of tank. I can't determine the brand of tank so here is the DIY idea.