Below is a quick diagram I did to show how I magnetize my carrying cases/boxes, movement trays and figures' bases. (Click to enlarge.) I've started to change from galvanized steel for the top of the movement tray to "paper steel" (I got mine from Renaissance Ink.) It's easier to work with.
I've also switched from basswood bases and chipboard/basswood movement trays to plastic, since the plastic is more dimensional stable, so everything's not warping so much with changes in temperature & humidity.