Replicated the excellent SKÅDIS Universal Hook so I (and you!) can use it in OnShape projects:

Skadis_Universal_Hook on OnShape

Magic numbers: 4.5mm thick, 35mm tall overall, 23.2mm wide, base is 16.5mm, hook is 5.2mm inside (SKÅDIS pegboard is 5mm thick).

Update Jan 2020:

If you prefer to use different CAD tools, I uploaded a .STEP version to save you a, uh, step.