Some pics of the heat sink and bracket that I'll be using during testing. The heat sink came from a junk PC, and the bracket is bent from 26 gauge copper sheet.
Heat sink and bracket, drilled and tapped.
Bracket mounted to heat sink.
Of course, something always goes sideways when working on any project. In this case, I planned to use an existing hole in the heat sink (already drilled and tapped) for mounting the 2SC1971 driver, but neglected to measure it first. As luck would have it, it's too large, so I'll have to fill it with JB Weld, redrill and tap. That's a project for another day.