I needed a place to put my geometry triangle ("Geodreieck"), and of course my IKEA SKÅDIS wall is next to the desk anyway.

The other models I found wouldn't work for me, so I designed this one. It can hold a standard geometry triangle up to 2.2mm thick (see attached pictures for mine), but if you want to fit a different size, you can download the attached FreeCAD file and adjust the parameters.

I designed this so it would print upside down without the need for supports, while still allowing a very nice and tight fit on the Skadis wall.