What about getting your end glass the same size as your tank end then building the blocks in only a couple of inches on that end from the side and bottom so the glass can rest straight onto them and be held in place by the water pressure (and silicone!) Then, when you build the extension and remove the glass the block work that was holding it will only intrude a couple of inches all round so not taking up too much space from the new tank size and there will be no screw holes left after.