A simple screw knob in two halves using M6 bolts.
Just superglue the halves together after inserting the bolt - you might need to pull it in from the bottom with a nut and washer to get the bolt head to seat correctly as the pocket is deliberately undersized to get a strong press fit.
30mm_M6_screw_knob_male_bottom_half.stl | 232.1KB | |
30mm_M6_screw_knob_male_top_half.stl | 202.8KB |