These are modular units to display your M2 Machines diecast. These will fit the cars that are still on the base and will fit with the plastic cover still on. To fit the diecast still in the original cardboard packaging you may need to upsize a few %.

Base module has a flat bottom, stacker modules will lock into the base and other stackers. Personal experience I've stack them with 1 base and 4 stackers and they fit on a standard bookshelf pretty well.