My take on a simple coat hook. I designed it to be relatively sturdy and easy on the clothes I hang on it. It takes two about-4 mm diameter flat head screws, with about-7 mm diameter heads.
coatHook.stl is the printable file; CoatHook.FCStd is the FreeCAD source for the hook, so you can study how I made it.
If your printer doesn't do overhangs well, you may want to print this with Support material.
Print on its side, so that the strong XY plane supports the load of the coat.
Drill two holes in the door, to hold the hook. Tighten the screws just enough to hold the hook. Don't over-tighten, or the hook will eventually split vertically.
I designed the hook to use a pair of spare screws I had lying around. Open the FreeCAD file (CoatHook.FCStd) to see how I created the hook profile and screw holes.
I chamfered (beveled) the left and right sides (top and bottom when printing) to give the hook a smooth outline, so it wouldn't dig into clothes I hang on it.
I made the hook part a full quarter circle because I don't like clothes slipping off coat hooks.
I rounded the top and bottom of the hook base so it blends into the shape of the door.