Just wondering if anyone has made or attempted to make their own GPS mount or does everyone use a manufactured one? Was thinking of trying to mount it on my stem as the stem cells I have mounted to each side would help stop the unit rotating around the stem.

You've got the etrex 20, IIRC? The standard bike mount for these fits on bars or stem, no problems. Pull the zipties tight enough and it won't rotate anyway. Only issue might be the viewing angle - the GPS is held parallel to the stem unless you pack it out somehow.

I've always just used the standard ones.

Section of old inner tube wrapped around your bar/stem prevents any movement from the mount.

I know a chap who 3D-prints them with glow-in-the-dark inserts for the central clip piece. Group-buy of a BB-spec if a few of us want the same thing?

They are only a fiver to buy one off eBay (or if you're prepared to wait for Hong Kong post to do it's thing, £1.57)...

Went for an eBay one, decided making my own wasn't worth it.