I just bought this, which comes with many different size collars so you could get one that sits at the base of the post and allows free travel. TUSITA Seat Post Quarter Turn Mount Compatible with Garmin Varia Rearview Bike Radar Tail Light RTL510 RTL515 RVR315 - Cycling 27.2mm 30.9mm 31.6mm Bars Seatpost Mounts - Bicycle Road MTB Accessories https://www.amazon.com/dp/B097XS7SFF/ref=cm_sw_r_cp_api_i_M9VTNVS899JAEMQ2S9ZC?_encoding=UTF8&psc=1
I just saw this that can mount on the seat post
TUSITA Seat Post Quarter Turn Mount Compatible with Garmin Varia Rearview Bike Radar Tail Light RTL510 RTL515 RVR315 - Cycling 27.2mm 30.9mm 31.6mm Bars Seatpost Mounts - Bicycle Road MTB Accessories https://www.amazon.com/dp/B097XS7SFF/ref=cm_sw_r_cp_api_glt_i_B79GF4KQQMQSPWJR1V0T?_encoding=UTF8&psc=1
Forgot to say, I purchased this for the radar: https://www.amazon.com/gp/product/B097XS7SFF/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
It is much more secure mount and the radar stays in place (have it on a Gravel Bike).
My Garmin Varia RTL515 is being held by this mount ~
There's a specific Blendr base for the Domane, make sure your LBS gets it in: https://www.trekbikes.com/us/en_US/equipment/cycling-components/bike-stems-accessories/bike-stem-parts-accessories/trek-domane-blendr-cable-wrangler-duo-mount/p/32459/
For Varia radar, this mount works well on my SL5: https://www.amazon.com/gp/product/B097XS7SFF
Do you already own the UT800? If so, then the gopro mount/extender may work: https://www.trekbikes.com/us/en_US/equipment/bike-accessories/bike-lights/bike-light-accessories/bontrager-blendr-gopro-mount/p/22248/?colorCode=black
Personally I went with the Ion200/FlareRT combo with the Ion200 on the Universal Light Mount and the Flare RT on the integrated seat post mount the Domane comes with.
Found it on Amazon
TUSITA Seat Post Quarter Turn... https://www.amazon.com/dp/B097XS7SFF?ref=ppx_pop_mob_ap_share
I got this one which is similar to the official Garmin version, but extends slightly outwards to maintain a more vertical orientation: https://www.amazon.com/gp/product/B097XS7SFF/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&th=1
This mount works great with my SL5. Secure and solid to the seat post.
Came today already and first ride with it. What a better mount lol, can't believe I waited so long