The parts were designed first as a single piece in FreeCAD almost fully parametric (adjusting numbers may or may not break the design). Then each half was created from the full design for the decoration.
I wanted to be able to press the action cam buttons from outside, therefore I made those parts of the outer shell flexible with cuts.
Decoration means basically wood-look. This part can be perhaps automated with textures but for now I simply used the sculpture mode in Blender. Sadly, any change to the model would require a new blender edit on STLs. I printed the parts with organic supports. You will need them especially inside and removal is slightly tricky.

2 parts are held together with strong magnets. I used a thin layer of hot glue for a water tight fit. I describe the workflow on my blog here: https://sarentasciyan.eu/how-to-simply-make-rubber-feet-for-projects/
Magnets are glues with superglue and then further supported with hot glue. For proper orientation, pairs of the magnets are in opposite orientation.

Also, I have placed 3 M3 screw ports with square nuts (I had them from my very first 3D printer lying around) for a tighter/stronger assembly.
I decided to paint the parts with acrylic paint, I happened to have. After printing in black PLA, I coated with transparent acrylic varnish spray first. Then I had to paint 2-3 layers of paint to get a good color quality. Finally, to protect it from water, I coated the hole outer surface with polyurethane spray, which also gave a nice glossy look.

And yet, no picture of Milli :[