Print in your preferred filament; I used PETG, but pretty much any type should work well.
You will need 4 8x3mm magnets (optional), and either glue with some scrap filament and clamps, or 4 M5x16 screws, depending on the version. If you don't have clamps and sill want to glue it, you can glue the screw version.
- Glue version: Use something that holds plastic well, of course. I used 5 min epoxy. I wouldn't suggest superglue, as you need enough work time to put it together and clamp it. You will only need a thin film, but make sure you get up near the corners. Use about 10mm of scrap filament in the small holes to align the halves for gluing. Make sure you align it properly! Make sure to clamp each corner to keep it accurate. Once it's dry, use a razor blade to shave off any excess glue.
- Screw version; use 4 M5x16 screws to put it together, again aligning it properly. Snug it down until the parts are flush with each other. You can also glue this version.