If you can find one through one of their partners https://eurodita.com/category/timber-sauna-pods-barrels/ are pretty decent.
These are pretty much standard
https://finnishsaunabuilders.com/collections/outdoor-saunas
and just Google "Finnish Sauna" and you'll see it's mostly shed forms and barrels. A few Kota shapes are thrown in.
http://localmile.org/blog/2020/11/05/notes-on-building-a-sau...
Basically there is not enough room in the bottom for cold air and the cold air Gradient is higher than it should be.
There are all sorts of other design considerations, that impact the convection and air flow that i dont think these ever even thought about