Made one of my old things into this customizer. Personally I use these to hook up towels in my bath room and jacket in my bed room. I believe most people have 30-32mm door hook so the stl should work fine anyway.
The "bumps" are to prevent slippage. But they only help when the door is closed. In the worst case scenario, anyway, when the fit is loose and load is heavy, you could always use some adhesives. Refer to the picture for what each variable means (the 'a' is an inner dimension, clearance not included).