Tee Wing Knob 1/4-20 Socket Head Cap Screw Bolt
There were not many non-metric choices so I uploaded this one for a 1/4"-20 (or 28 I guess) SHCS. You will probably need some glue or a harder material than PLA. I tried just a press fit in PLA and the hex does not grip well enough and spins.
End cap also included if needed.