I have an idea. If the design is deep enough, make small (1"x1", or even4" wide?) removable stick supports to place in the middle of the larger openings in front of the tubs. That way, these will support the long shelves and prevent them from sagging or resting on the tubs over time. Simply pull the support out whenever you need to access the tub, then put them back whenever you close the tubs. It's simple, cheap, and I don't see why it wouldn't work and prevent sagging. Actually, if you took a short section of that 18" wide melamine and cut it to the height of the tub, there is no way it could sag over time, and it would give the snake in the 41's some privacy, too.