These hooks were designed for hanging a 1 x 1/2m framed mirror on a door. There are 4 variations of the hook, one for each corner of the mirror. The hooks screw to the back of the plastic mirror frame, and are slotted to capture the heads of mounting screws in the door.
The doors I am hanging on are hollow and the mounting screws only pierce the outer timber skin. Even so, 4 screws seem to be adequately strong to support the mirror.
The brackets were printed on a Replicator with ABS at 25% infill, 1 extra shell, no raft or support required.