its a 45g 36x12 inch footprint.
i may not need the diaginal support. its mostly to keep the stand from twisting or bending side to side, but i'm going to put plywood on the top and bottom and as a shelf in the middle and screw it into all the sides, so that will help with diaginal bend. i'm not putting a shelf on the bottom, i'm actually putting a piece of plywood on the bottom of the stand that will extend about 4" farther all the way around to add stability.
also i may end up just covering the whole back with plywood, since if i buy a 4x8 foot sheet, i will have a good chunk leftover after the top, shelf, bottom, and doors. if i attach a piece of plywood to the back, that will also help a lot with any twisting.