OnePlus 3 Frame/Cover for External Use with GoPro Mount Style
thingiverse
I did it for my OnePlus3 and the internal size is for OnePlus3 + Ringke Cover, which you can find here: http://fave.co/2eiSmRx - but it will work with other similar covers too, obviously. To finish it off, you'll need 4 magnets that are 4x2 mm in size. I recommend printing it vertically (check out the picture) at a resolution of 0.15 or 0.20.
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