I loved the Rugged box that ORDEEP remixed for the Osmo Action 3, but I have the original Osmo Action which has different dimensions. This insert is designed to be printed in TPU and have a soft grip on the camera. For that reason, it may not work in a non-flexible filament, but I haven't tested that. 

The insert slides easily in and out of the case with only a small bit of friction, so you may want to glue it in place if you're looking for a tighter fit. 

I'm also including my Fusion 360 file so that you can tweak dimensions if you'd like. Please forgive my sloppy dimensioning, parametrizing, etc., I'm brand new to 3D modeling. You should be able to rework it for other Osmo versions. I'd sketched a dust seal first, and based the insert dimensions from there. The seal is visible under “Body 1.”

I used the Bambu P1S with Polymaker TPU 90 in Grey for my print. 20% triangle infill gave it about the right amount of “squishyness," though you could probably go a lower. I did have quite a bit of stringing, but that cleaned up easily and just leaves cosmetic blemishes. Still working on my TPU print profile.