furniture hook designed to fit on 19mmx19mm vertical profile. - profiles of IKEA FJÄLLBO rack in my case.

one wall is thin so it can bend to fit around the profile at any height. on the opposite side there is a hole for one M4 screw to tighten the hook so it wont slide down.

I printed mine with ABS with triple perimeter at 30% infill so that it definitely wont crack under heavy load. no supports and no brim.
model can be very easily resized to fit on different profile sizes.