So I went to make a chin mount for my NXR 2.
Started with a look online and didn't find too many files out there.
Kind of surprising as I'm not the first one to do this!
Found one but it was pretty rough CAD and wrong angle... other than that it was great!
I designed my own and with a J-mount clipped in, my Osmo Action 4 is at a great angle (ultrawide camera angle and the fuel filler is partially in shot).

I printed it with 3 walls and 30% Gyroid infill on my Bambu A1. I print slow for accuracy and strength (25mm/s outer walls and 50mm/s inner walls).
Model is a perfect fit to my Shoei NXR 2 (no rocking).
I used Scotch VHB mounting tape from an action camera attachment assortment pack.
First I sat the finished print on the helmet in the mounting location I wanted and traced lightly around (and marked the rubber at the rim) with a graphite pencil.
Then I sat my mount on the VHB, traced around it, cut it out, then stuck it to my model (first I rough sanded the 3D print on the mounting face with 240 grit then wiped the print clean with IPA).
Next I cleaned the helmet with IPA (kept the pencil lines faintly there).
Using a hair dryer I warmed up both the helmet chin area and 3D print (with VHB on it).
Pressed the two together with good force and held it for a minute.
Let it set for an hour, then she is good to go!

Print needs a little bit of support. Mine broke away clean and then clean-up was very minor. Minor hobby knife carving to deburr the build plate face.
Using a lighter you can melt (carefully!) any stringing or stray bits.

STEP file uploaded too, in case you wanted to modify it easily for a different angle or whatever.