On Monday, August 17, 2020 at 10:47:50 PM UTC-7, John Doe wrote:
You know, like the chuck on a metal lathe but smaller.

If you just want a clamp over a rod, weld (or otherwise affix)
a shaft to a black-iron pipe cap (or other cup-shaped object)
and drill/tap three holes in the cap, at 120 degree separation,
so that three bolts can be tightened on a workpiece. Crude, but
effective (the shaft fits into your drill's three-jaw chuck, of course).