I wanted a case for a small MP3 player, with magnets in the back so I could stick it to my toolbox and to the dash in my old truck. It took four iterations in FreeCAD to get the dimensions and openings right, and my Bowden style printer doesn't print TPU very well, but the final result works for me. 

It's designed for a very specific little no-name MP3 player, but I made it parametric so it can be fairly easily modified in FreeCAD. I'm including the FreeCAD file - you can just change the values in the mp3_case dynamic data at the bottom of the list. The magnets are some I salvaged from something a couple of years ago, and are glued to the inside of the back.