When making jigs and fixtures, it is always a chore to find hardware of the correct size. We have knurled knobs with more robust 5/16-18 threads for increased strength and durability.
For a custom thumbscrew, press-fit (vise assisted) the socket head of a capscrew (sold separately) into the knobs. Use nylon washers (sold separately) as spacers or when you have to apply pressure right at the capscrew head.