After fighting this problem for years. The fact that those M2 acrylic cases aren't designed to stack...maybe the intention was there but it never worked for me. I called the event an "avalanche" when the cases tumble to the floor breaking the cases and sometimes damaging the cars.

I solved my problem ordering these 3/8" x 1/8" felt protectors from Amazon. There are 1000 per roll at $19.00 so I ordered 4 packs. I have nearly 1000 acrylic cases and 4 per case is required.

Sticking 4 discs just inboard of the thickness of the plastic is enough to securely stack my M2's. Yeah!