I have seen various board game bit trays on Thingiverse. Some are magnetic and some are stackable. I wanted bit trays that are magnetic and would tesselate so as to take up minimal space on the table. I also wanted them to be stackable to make them easily portable to pack in my games box when I head out to play games with friends. This is the design that I came up with.

There are 3 sizes. All are 25 mm high. The small tray is 52.3 x 52.3 mm, the medium tray is 74 x 74 mm and the large tray is 104.7 x 104.7 mm. I have only printed and used the medium tray so far and it seems to be suitable for most needs. The reason the medium tray 74 mm along each edge is that it is just slightly wider than sleeved 2.5" x 3.5" card which makes it nice when you have to put cards beside the boxes.

Each tray uses 5x5x2 mm neodymium block magnets which you can purchase cheaply from various places such as AliExpress and eBay. They fit snugly into the trays with a bit of pressure, but for best results, each should be held in place by a drop of superglue. The tray should have opposite sides with the same polarity and adjacent sides with opposite polarity. One pair of opposite sides is marked with two spots next to the magnet to help you easily identify which sides of the tray will go together. Sides with the spots should be attracted to sides without the spots. For best results, test that your magnets are the right way around before you glue any into place.

I have also included versions of the tray in each size that do not use magnets, just in case anyone wants to use this as a basic stackable tray.