I made a custom fold 3 case (back only) for magsafe and quadlock
Galaxy Fold 3 Magsafe+Quadlock case (draft) by natemiller82
https://www.thingiverse.com/thing:6354529
This is a mashup of two other models I found on Thingiverse - https://www.thingiverse.com/thing:5704716 for the Magquad lock mount, and https://www.thingiverse.com/thing:4935721 for the Galaxy fold 3 case. I have not yet tested this design, but the it's meant for the quadlock magring to go in the open circle, so it can be compatible with magsafe and quadlock mounts.
The folks in 3d printing helped me out greatly with this, and I was able to mash this together today. I haven't printed or tested it yet, but it's designed to be used with the quad mag ring. I'm going to use some very thin and strong 3m adhesive tape I have to ensure it is secure to the phone, and use for a quadlock mount on the bike if it actually works.
Perhaps this niche case will be useful for someone else?