This is a heavily remixed and remodeled version of a hat mount designed to be run on a hat in reverse but keeps it more stable.  I've been running this print for the last ~3 years and it's worked great for me. I print out of PLA, then use boiling water to soften the part that goes over your head and formed it to my head shape for comfort. Uses standard gopro/dji nut and bolt. Designed to hook onto the backstrap of the hat, and then the “lever” piece goes between your head and the hat. May not work with hats with strap adjustment, my hat is velcro adjustment. 

Step included if you wish to modify.