I would also say 13-17 cm is optimal. However, the support on the upper edge of the door (frame) is also important. This should also not be smaller than 1.5 cm.

I cannot measure the thickness of the wall. The door frame can be 18 cm deep. But not more. 21cm is too much. Apparently built for American board shacks.

Gemäss Website 6 inch = ca. 15cm. https://harbingerfitness.implus.com/products...