I've included two sample STLs for a hook that work on the wire shelving we have around our house. One set of shelves we have are about 24.5mm high and the top and bottom bars are about 5mm in diameter. The other set are 31.0mm and 6.3mm. These two measurements correspond to the ht and gap parameters respectively. The parameter list is as follows:

• w - the width of the hook; you'd normally want to keep this smaller than the spacing of the rods across the top of the shelf

• th - the thickness of the hook; increase this if you need a more robust hook

• gap - the width of the gap in the hook's clip; corresponds to the diameter of the top and bottom bars

• ht - the height of the shelf as measured from top bar to bottom bar, inclusive

• l - the length of the hook, from the top of the clip to the bottom of the hook

• r - the inside radius of the hook; if you have something with a solid handle, set r to a bit more than half the handle's diameter

Printing

Nothing special here. The hook will print without supports. It's probably best to use PETG or ASA. PLA will tend to deform under load.