I found Penthon Tools on Ebay selling a drill attachment that fits some
sizes of wire nuts. I next
purchased a Klein cross tip crank-style screwdriver (it has a wider shank than
the flat head), cut it off, and attached it to the wire nut spinner
using two steel bushings and JB Weld.
holds up well and greatly reduces the time and energy of twisting wire
nuts on by hand. Overall cost was about $15 each.