Inspired by https://cults3d.com/en/3d-model/various/varia-radar-seatpost-mount-for-canyon-aeroad
Use the original Garmin mount, you can unscrew from the RTL 51x mount.
Uses 4x M3*12 Hex bolts
Uses M3 X D5.0 X L4.0 Hotmelt inserts https://www.aliexpress.com/item/4000232858343.html?spm=a2g0o.order_list.order_list_main.127.359f79d24tfHXZ
UPDATE
I printed another one at 101.6% scale the fit is slightly more forgiving. It may be a question of calibration of the printer.
The author remixed this model.
Uses 4x M3*12 Hex bolts
Uses M3 X D5.0 X L4.0 Hotmelt inserts https://www.aliexpress.com/item/4000232858343.html?spm=a2g0o.order_list.order_list_main.127.359f79d24tfHXZ