Only other method I can think of would be to just buy a larger O-ring, since the provided ones are way too small for the aero seatpost.
Curious to hear if anyone else has another solution (or the size of O-ring to buy), I saw a slowtwitch article where they mounted the Varia to the integrated rear hydration mount in the seatpost, looked cool but then you lose the rear bottle…
KOM Cycling for Garmin Varia Mount Tail Light Models Including RTL515, RTL510, RVR315 Featuring an ATOP Dial for Security - Varia Seatpost and Under Saddle Mount - Fits Most Seatpost Sizes and Types https://a.co/d/0amhrtF4
Mounts under water bottle cages on seat post. It is awesome.