Just another carriage bolt knob, this one is for 1/4".
If you need one for 3/8", I made one here: https://www.thingiverse.com/thing:2759151
There are 2 versions, v1 is the "stubby" one, no supports needed. v2 is the more traditional knob, you will need supports for that one.
UPDATE - I just learned how to add knurling in Fusion360, I decided to practice on v1. It turned out okay so I uploaded it (1:4-knob-v1.2).
25% for low torque applications, but you're not a dummy, so you know to beef it up if you're going to be putting a lot of force on them!