I think this will be fine I would install corner braces and build a removable timber centre brace. If you cut a piece of timber a little wider than the tank with a groove at either end to slip over the glass. Then when you fill it with water if the sides flex you won't be able remove the timber brace and it won't smash the tank
I've been thinking about that. But wood flexes, and I'm picturing the wood either cracking or literally popping off. A bonus would be that wood also compresses some, so it wouldn't create hard points against the glass like a more rigid material would, if there were any inconsistencies along either surface (there probably would be).