This is what happens when "good" solutions aren't cheap enough: it's an off-the-shelf 12V 1A geared motor (designed for brass case preparation), clamped to the drill's drive shaft with a rigid coupling.  On highest reduction pulleys the chuck is turning about 50rpm.  Beauty in kludginess?

[![Kludge][1]][1]


  [1]: https://i.sstatic.net/8xfsx.jpg