Seatpost Mount with a case and leash in case it falls off during a ride.

I have something very similar and love it.

LICHIFIT Bicycle Saddle Support Seat-Post Mount Tail Light Holder for Garmin Varia Rearview Radar / RTL510 https://smile.amazon.com/dp/B0832D9DMW/ref=cm_sw_r_cp_api_glt_fabc_4WN0X9M5XWDP9J4VX5MN?_encoding=UTF8&psc=1