This adapter allows to use any common filter adapter on the GoPro Hero 3 housing.
I took a 37 to 55mm filter adapter and cut out the shape of the GoPro housing lens by using the template (the thin part of the 3D-model).
Also drill the screw holes with 2mm diameter and countersink them.
Drill out the holes in the printed adapter to 1.5mm and tap a M2 thread. For gently fitting smooth the inside of the adapter with fine sandpaper.
Use M2 countersunk-head screws to assemble the parts. Done.