UPDATE - I've added a second file, with some advice from a friendly Redditer (u/jjreinem, thanks!) in which I have performed some rounding on the edges of the dice tray. I also applied a heavier rounding near the corner on the rails to provide better clearance in there.

This is a corner dice tray designed to work with tables with magnetic rails. The tray is just under 22.5cm square for maximum dimensions. It is 7 cm tall. Supports are needed for the rails. The rails have three holes each in them - design to hold a maximum diameter ring magnet of 12mm with a thickness of 3mm. A pilot hole for an M3x0.5 screw comes pre-modeled in the STL. For the prototypes pictured I used these magnets: https://www.amazon.com/dp/B09Y8JC9CH

The test print is designed for a quicker print to test out the square edge that makes contact with the table. It is not necessary for the final product.

This is still a prototype. The magnets above are not super strong so they do not "snap" to a metal rail. However, the tray does not jostle free when the table is knocked or the dice rolling gets particularly salty. Also, you could probably spray paint it a bit better than I did or even so some sanding. My desire for perfection is often low.