[enimodas]

He's not using a normal sauna, but something with heaters/lamps. I'm not dedicated enough to try it myself: https://www.reddit.com/r/PsoriaticArthritis/comments/es5w6f/...

[detaro]

There also are commercial "apartment saunas", which are basically a tent with some heaters ready-made. Not sure if they fit the specific requirements here, but less improvisation.