Update 2: I've added multiple versions with slots for the camera cable and display cable. This means that now all possible usage combinations for GPIO, display and camera can be used. The .STL files indicate what slots are present in that design.
Update: I added a second version of the top part of this case, which has a cut-out for those who want to use the GPIO pins. The .STL file is included with the thing files. There's also an extra image that shows the cut-out with the official 3B+ technical drawing as an overlay, on which I based the location of the cut-out.
This design is based on 0110-M-P's design: https://www.thingiverse.com/thing:922740
There are several changes in this model (compared to the original design):
The fourth image shows exactly what hardware you need to mount the Raspberry Pi and to close the case.
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The author remixed this model.