Here is what we recommend to mount the Garmin InReach Mini:
1. Buy the Garmin suction cup accessory for $29. On the Garmin site it is: https://buy.garmin.com/en-US/US/p/pn/010-10851-30
2. Set aside the Suction Cup section of that and keep the part that the Garmin InReach slides into. That item has a 17mm socket to receive a 17mm ball.
3. Buy the 17mm-20mm adapter from 67 Designs and push that into the 17mm socket on the item in #2. Get the adapter here: https://www.67d.com/collections/garmin-compatible/products/17mm-to-20mm-connector-for-carbon-fiber-arms
4. Buy an arm from 67 Designs. Most buy a Mini size arm for the Garmin InReach, but the choice is totally dependent on your setup and personal preferences. Select from the Carbon Fiber Arms menu on our website. Too many options to link here.
5. Buy a base from 67 Designs. In your case, buy this BT55 20mm trackball for the Series 55 Rail. https://www.67d.com/products/series-55-track-ball
That's it. Really simple installation. Sorry to send you to another vendor for some parts, but we want to support another American company - Garmin - and not have to resell a non-US made knock-off. Deware the fakes and knock-offs. Support US innovation!