Here is how we are building the 100% natural tub in the traditional way.
The shape most suitable is the “TARU” which is assembled like a barrell with vertical planks of hinoki. The water pressure distributes uniformly on the perimeter and is collected by the stainless steel bands hammered from the bottom up.
The hinoki planks are cut with variable angles so to match eachother seamlessly. Between these we used a natural bond made of rice starch called “SOKOI”. This is 100% natural and does not contain anything but rice and water!