A hook that fits Ikea's Elvarli wardrobe system T shaped side rail, to extend the storage space a bit.
https://www.ikea.com/us/en/cat/elvarli-system-35766/

Print settings: 
I used PETG, 4 walls and 25% grid infill, and it is definitely strong enough for jackets and bags. 

Tolerances are quite tight, to not slide around on the rail, but you can play with x/y hole compensation in the slicer, if needed.

Mounting: 
Simply remove the rail, slide the hook on and place the rail back