I actually encourage you to put the Varia lower on the seatpost like Tadaka has it so it can detect cars when another rider is behind you. My second video ever(not great) I did that with the older varia (Where to Install Your Garmin Varia Radar When Riding with Others - YouTube).
I have used it with a Giant D-Fuse seatpost like you have it but with the round rubber insert and it has worked well for me but I am looking to do something different. Here is one on Shapeways but kinda pricey Tarmac Seat Post Varia Mount (NSM2KQF64) by MadBikeSkills.
For that price, I would probably just use some sort of foam to take up the space between the flat part of the seat post and the attachment. I will do a little more digging a post back if I find something.