While it may not be cool...
I have securely attached my ay-up mount and lights to the underside of the lower bar on my Grail. It has a flat profile. I did put small strips of rubber under the mount where it meets the bar. Couple of zip ties and BAM... light.
The Garmin Varia is perfect for any seat post. It comes with a mount that'd fit an aero seat tube as well as the one I use for a round seat post. Besides the bright light and solid mounting, it's radar feature is second to none. Best gadget I've bought ever for cycling. It's just that little bit of peace of mind on the road.
Hope you figure something out 👍