Here's a hook that I printed to give us extra towel/robe hooks on the back of your door, without having to drill or use removable sticky hooks.

Simply print, and slip it over the top of the door.

It is 3mm thick where it goes over the door, so it won't work if the door seals very tightly. Works for sliding or hinged doors.

The hook will fit on doors 35mm thick (minus printing tolerances). If your door is thicker or thinner, simply scale it up or down in your slicing software, i.e. scale = ((door thickness + printing tolerance) / 35mm) * 100%