Simple shelf hook that you can customize to get the right width, deepness, thickness etc.
Customize and print for your special case. At the gallery black hook is printed with default settings of 0.3mm layer and 3 perimeters and the orange one with 0.3mm and 2 perimeters.
Basically depends on the weight of the "thing" to hook - you can pick the right attributes for your own customized thing at Customizer or locally at OpenSCAD.

No special instructions needed. Open Customizer and create your own version. OpenSCAD source is attached so feel free to play with it as you need.
if any questions - please ask in comments.