Simple and reliable bench on shkant and self-tapping screws. All details it is visible in drawing, all 8 shkant and 12 self-tapping screws are required. Pay attention that the expansions of legs (replacing struts) fasten to legs self-tapping screws through grooves from above. And to a seat this construct fastens from a reverse side.
Still there is one rule concerning what has to be the seat in a steam room: between whetstones it is necessary to leave gaps without fail. Gap width in 1-1,5 cm is considered optimum. It is clear that they are necessary for running off of water and acceleration of drying.
Those benches on which will lie paruyemy, have to be rather long that it was located to the utmost, and also rather wide that it was possible to put freely hands along a body. Too it is better to measure such things on specific people.
And here option in general the elementary and even slipshod which can be made even of moisture resistant plywood. Of course, you will not put such bench in a sweating room, but it quite can fit somewhere in a waiting room. As on the drawing fastening is not shown, we will risk to suggest to fix a design usual corners.
The ergonomics are a science how to do things and processes convenient for the person. In this case it is about that seats in a bath did not give an inconvenience the sizes.
Here kind of the first drawing. It is not simple a bench any more, and the whole plank bed. At first the seat from the ground boards of 50100 mm which is fastened with two cross whetstones prepares. To whetstones legs fasten, and instead of a usual strut or a strut the framework from above (at once under a seat) is sheathed by a board of 25150 mm on perimeter.