You would have to recess the bottom allowing room under it for the levelers, so the bottom piece is 1.5" shorter than the top piece. The back piece would stiffen it and help carry the weight without warping the bottom, but in front it might warp unless you had a vertical piece behind the door on both sides. Your design is certainly simpler and less apt to warp over the months the tank sits on it. But, you might have to do a lot of shimming to level the top, but if the shims are just under the vertical side pieces no warping should occur. That is one problem with MDF, and to a lesser extent any other form of wood that you have to consider when you plan to keep a heavy load on it for a long time.