For my miniatures, I attach adhesive sheet magnet on their bases. They stick to these boxes with no trouble.
The primary box that I use is the one for copier paper (11" x 17"). Cut down the sides to the height you need.
The first trick is to cut the sheet metal. This is just ordinary roofing metal that you can buy in
rolls at the big-box hardware store. Notice that I use a paper cutter! This makes a clean cut.
Cut to the correct size, the metal will fit perfectly. There is nothing keeping it there except gravity.
Note that it is flat. You can easily flatten the curled metal on the edge of a table.
Here you can see it on edge, with a large metal figure (with a magnetized base) stuck on it.
Look at how many figures it holds. These boxes can really store a lot because there is no foam.
One of the cool things about this system is I can write the contents on the outside of the box
and easily find it stacked in the closet. You can also see how this box was 'repurposed'....