I couldn't find a hook for my keyhole shelf so I designed one. It has dimensions of 13.2 x 55.0 x 61.1 mm. It is very sturdy. Just don't hang from it, put pressure on it, etc. The file KeyholeRackHook.stl is the hook. The KeyholeHookBase.stl file is just the base for these shelfs and you can create your own.
Print with support option "Touching Buildplate". Lay it on it's side when printing.