RTL515 tether loop

The 315 doesn't have a tether either but I've tethered the spare Garmin quarter turn mount I use to attach the radar to my saddle bag. The single elastic band worries me too

I used next to the rubber band tie-rips to secure the universal mount to the seat post. 
Between the seat post and the rubber pad that supports the universal mount I applied double sided tape to reduce any twisting/turning of the universal mount around the seat post.
The universal mount is now firmly fixed and bumps are no issue.

Fizik Saddle to Garmin Varia UMS - Bup Labs

I also used a zip tie through the seatpost mount to secure the mount to the seatpost.  Im just worried about hitting bumps and the device coming off the mount.  

I like that solution!  I'm sure I have one of those cords laying around someplace.

and the tether on the zipper can be also looped around saddle rails instead of saddle bag

Great idea/solution!

bravo, clever

As far as the mount goes I bought this one for under the saddle. Solid as a rock. 

https://www.amazon.com/gp/product/B08FT7GBSK/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1