The back walls of some drawers in my kitchen kept popping off, so after a couple of minutes of design and 13 minutes of printing - a simple corner bracket. It won't support any heavy weights or anything like that, but it should be good enough to stop the old drawers from falling apart. It should be "good enough" for a bunch of other stuff too.

Overall dimensions: 20x20x20mm Wall thickness: 2mm Hole diameter: 3mm