The is a remix of the foldable backpack hanger from jackw01. I removed the carabiner gate, opened up and rounded off the hook. To add grip to the hanger, I add a few coats of liquid electrical tape to where it comes into contact with the flat surface it is resting on.

The original design by jackw01 is set up for a M5 x 18mm bolt and an M5 nylock nut (fits great into the existing counter-bore). The two parts can be scaled in the Z-direction (leave X and Y alone) to adjust for shorter or longer bolts. These ratios worked for me for the given bolt lengths:

111% - M5 x 20mm bolt 100% - M5 x 18 mm bolt 88.8% - M5 x 16mm bolt 66.6% - M5 x 12mm bolt