This is a narrow field of design. I had some problems to place my GPS (Garmin with 17 mm ball attachment) in a good spot on my motorcycle. I didn't want to do any major intervention in the drivers area and I saw this opportunity in the fork mounting details.
There is a hole for a 12 mm allen key that was suitable for this. The only negative is that you have to look down to see the GPS so be careful when you are driving.
If your GPS has a different attachment it will of course not work, so you have to redesign. If you want to redesign it for something else, for instance your phone: the hexagon part is slightly conical from 11,7 mm in the bottom to 12,3 mm in the top. That's the trick to make it fit and stay tight in the hole for a long time.
