I still use my old eTrex as a back-up to my phone when out hiking - when conditions deteriorate, the phone is stowed, but the eTrex soldiers on...

I grabbed the clip / mount from enif's handlebar mount, thickened it, and added tunnels for < 3mm-wide zip-lock ties (for pack harness attachment).

Photo shows unmounted 'thing' on the right, and a harness mounted copy of the left.

I printed mine in 'impact modified' PLA for strength and 'slipperiness'.