I created this hook to fit my 18mm shelves and hold my headphones with generous space around them. The opening is 24mm and the final resting place's height is 30mm.
The whole model is a 6mm crossection with most of the profile fileted. It's best printed laying on the side. No filets on the sides allow it to be printed without supports.

I recommend concentric bottom and top layer fill pattern and 15% cubic infill.