There's similar questions online that were asked.
Is there a way to lean a mirror/board against the wall and ensure that it doesn't slip without hammering a second piece of board on to a wall top of it? I've tried learning this mirror on a piano and for some reason that works, but I need a solution for a home without a piano. I can't afford making holes in my wall. This is also unwieldy because that means my mirror is fixed and I have to re-hammer in a board every time I move the mirror.
I don't trust grips on the bottom of the mirror to preventing it from slipping if I touch it. I don't trust corrugated rubber matts or changing the carpet.
Best Answer
Your cinder block holding a board provides an indication that the answer you seek does not have to be particularly elegant. You could place another board under the block, with the same cleat, positioned outside the base of the mirror. This would prevent the mirror from sliding away from the wall.
A less bulky alternative would be to keep the cleat, extend it wider than the mirror and attach it to the bottom of the vertical board with strong cord. This would allow the entire assembly to be collapsed when needed. It would be necessary to implement additional fastening to prevent the cleat from riding up the mirror. A slot in the cleat into which the mirror would seat would serve equally well in that respect.