A knob

Simply as it is, it's just a knob.
I originally designed it for the Improved bracket for RV or camper fridge shelf (https://www.thingiverse.com/thing:5515562).

Screw it on top of a hex bolt to ease hand tightening.

BONUS! Included Solidworks file so it can be modified to fit your needs.

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Consider offering me a beer. Beer fuels creativity and creativity creates good STLs :-)