IKEA Ivar Rack Clothes Hanger

The normal version requires a 16mm M3 bolt to secure everything in place. Just print the model together; reserve the “filler block"; put the hook over the horizontal side rail of an Ivar shelving unit; hold the filler block in the bottom gap and screw the bolt from the back of the hook and through the filler block.

14mm bolt may work as well, 12mm is too short.

There is also a “trivial” version that just snaps in and is easier to print.