Barrel saunas

A barrel sauna is built from staves laid lengthways into a cylinder and held with steel bands, a bit like a cask on its side. The round shape means less air to heat than a square room of the same floor area.

They're compact and recognisable, and are often sold as kits. The trade-off is headroom: the curve cuts into usable space near the walls, and a tall person may only stand upright down the middle.