Pannier rack for larger diameter seat post
It's probably 36&nbsp;mm; that seems reasonably common. Most seatpost racks go up to 31.8&nbsp;mm

Taking 36&nbsp;mm as a guess makes searching much easier: it's an upper limit for some models:

It seems easier to find a trunk bag attachment for such a large post. There is such a thing as a Rixen &amp; Kaul KLICKfix Caddy Post and Bar Clamp 2016.  That would allow you to mount a handlebar bag on the seatpost, or this Vaude mount is designed for their trunk bags but could be used with other things.  None of these are of much help if you want to choose a pannier to suit you.

Traditional style racks do exist, such as this Atran Velo Luggage Carrier Active Seatpost (which seems to have curved sides, but many panniers would still fit, and it's quick release). The search that turned up that last one (and the Vaude) was seat post rack "36mm" -rixen