I was sick of tossing my coat and dry cleaning on the back seat so I designed this hook to wrap around the roll bar padding which is just about 3" diameter. It hinges open to slide around the padded roll bar, then when something's hanging from the hook it's locked closed and can't fall off.
The hinge at top is sized for a 6mm bolt, but use whatever fits.
Edit 2018/09/04: I added v2 of the parts which includes a hole at the bottom of the ring because my hook wanted to spread apart when it wasn't carrying any load. The hole is sized so you can use a 3mm tap (or just ram a 3mm screw down its throat, savages). Also I recommend printing your parts in ABS to withstand the summer heat inside a rig with its top on.
I bumped the infill just knowing these parts would be load bearing. I don't know how much is sufficient.