comments:
outdoors_life22 posted on r/nfa3w
Somebody recommended these. Might have to stack a few https://www.amazon.com/TACFUN-Steel-16-24-TPI-Grendel/dp/B0DH9517LH
shakinandbreakin posted on r/longrange3w
jam nut could be an option. I'd definitely check alignment with whatever you throw on the mount.