One idea...have you thought about inverting the plexiglas to angle down to the right side of the tank against the right back corner, rather than up against the back wall angled down towards the front? If you switch it you would only have to put in one crate wall because the down-ward angel to the right would hold the gravel in place easier without two restraining walls. Plus, you would now have usable swimming space under the angled plexiglass that you could turn into a cave or something for fish instead of that closed-up, unusable space with the two walls.