A sauna backrest is a slatted panel that leans against the wall behind the upper bench, and it is probably the best value item in a cabin. A hot vertical wall against a bare back is the thing that makes people sit forward hunched, and a backrest fixes it for the price of a few boards.
Why the wall is the problem
Cladding at head height sits in the hottest air in the room and it is a flat surface in direct contact with your skin. Leaning back against it is uncomfortable within a minute, so people perch forward, which is tiring and stops them relaxing. A slatted panel with air behind it stays far cooler and spreads the contact.
Angle and height
A slight lean back, in the region of ten to fifteen degrees off vertical, is what makes the difference between a wall with slats and something you actually want to rest on. The panel should reach from just above the bench to shoulder height for the tallest user, and should stop short of the ceiling so hot air is not trapped behind it.
Timber and construction
The same low density knot free timber as the benches: aspen, alder or cedar. Slats with generous gaps, fixed to two vertical battens so there is an air gap to the wall, and every fixing from behind so no metal can touch skin. Rounded top edges matter more than they sound, because that edge lands between your shoulder blades.
Making it removable
Hang it on cleats rather than screwing it to the wall. It lifts out for cleaning and sanding, the wall behind can dry, and you can move it if you change the bench layout. Most owners who build one fixed end up wishing they had not, because the surface behind a backrest is exactly where a cabin gets grubby.
Questions people ask about sauna backrest
Do you really need a backrest?
Need is strong, but it is the cheapest change that makes a cabin comfortable. Bare cladding at head height is too hot to lean on.
What angle should it be?
A slight lean of roughly ten to fifteen degrees off vertical. Flat against the wall gives you much less of the benefit.
Fixed or removable?
Removable, on cleats. The wall behind it is where a cabin gets grubby, and a panel you can lift out is far easier to keep clean.