Teenage Engineering OP-Z synthesizer and sequencer is a small beast for musicians. As it's a quite valuable thing, I designed this very simple case for all of fans with 3D printer somewhere around.
I recommend the top cover (lid) to be 3D printed from flexible and not abrasive TPU (Shore 95A or 98A). It holds on thanks to friction. The main case has to be rigid, my is made from CPE.
Video – how it works: https://youtu.be/NomjaG6TN-8
Note: I've prepared also the hard cover (lid) which can be made from your favourite rigid material. As you do not want to scratch top buttons of your OP-Z on the go, choose less abrasive filaments for the lid like Nylon etc. Keep in mind that each 3D printer has its own tolerances and the lid is held by friction, it may be necessary to fine-tune the scale of the lid in the slicer a bit, if both of parts are too loose. Or use a sand paper to find tune them if there is a space too tight.
[19. 1. 2019] I have reuploaded the flexible lid.
[30. 1. 2019] According the response the rigid variant is viable too. :)