I’ve been buying 1-2mm thick plasticard, and sheets of fridge magnet material that has adhesive on one side. I cut the plasticard to the size I want and then glue down the sticky side of the magnet sheet with a little CA glue for extra hold. I have rare earth magnets in the bases of minis, and while the fridge magnet material isn’t as good as sheet metal, it’s way easier to cut at the craft table and holds the minis reasonably well.
I’ve been cutting them flush lately so that I can add on extra sheets to the back or side as needed when reforming, with a lip or border around the models this can’t really be done.
I can crank out large numbers of various-sized movement trays, either with extra space for a lip around the border or make it so that it’s completely flush with the base of the minis. I can whip up a dozen of these in maybe 20 minutes and then I run a black Sharpie along the outside edge of the plasticard so that it blends in with the edge of the fridge magnet material and almost disappears under the base of the minis on the table.