Congrats, Paul, on a cool running rectifier!
I've seen quite a bit of info regarding the aluminum/copper choice of heatsink. The best I can figure, and it seems you do as well, a copper sink to suck up heat with aluminum fins to disperse it might be best.
So, at the start of the day yesterday, I was getting perfect readings on my charge. 14.2v at 2000rpm and above.
At the end of my 4mi commute to work the rectifier felt rather warm. On the 'how-long-can-I-touch-it scale', it measured 2-3 seconds on the epoxy right in the middle of the terminals.
Doing the same test after my ride home the rectifier was barely warm at all. Checking the charge voltage, it would only get up to 13.5v or so all the way up to 4000rpm.
Not done yet...