There are saddle rail mounts like this HUAYUWA Bicycle Saddle Support Metal Seat Post Mount Tail Light Holder Fits for Garmin Varia Rearview Radar / RTL510 https://www.amazon.ca/dp/B08FT7GBSK/ref=cm_sw_r_apan_i_N2AZWSA6C6BAKH0TMJJP
It may work for you. One warning though - I have one for my gravel bike, but it mounts right tight under the saddle and the flexy shell of my phenom hits the switch and turns it off, etc. Depending on your saddle you may be fine
Using this one Varia mount - but i don't recommend it because its too loose and tends to break already
I have similar to the ones mentioned. One thing to do is blue loctite the two screws. Mine fell apart after a half year of use but luckily I was able to find my beat up Varia. Now I use loctite.
Mine kept moving because I had an aero seatpost. This mount solved that issue.
Yea, I got this https://smile.amazon.com/gp/product/B08FT7GBSK/