I store most of my miniatures (which are on steel or plastic steel bases) in magnet lined boxes. The slow part of any expansion has always been gluing the magnet sheets down without getting bubbles or having the corners curl up. I had a blinding flash of the obvious the other day: some sort of double sided tape would probably work well enough, since it doesn’t have to hold especially firmly. However, a little searching around led me to pick up some carpet installation tape. We’ll see how permanent it is, but at least sticking down a sheet was a task of only a couple of minutes.