Here's an update. After considering all of the cheap SCR controller boards on eBay, I decided that it would be more involved than I had hoped because I would have to buy and populate an enclosure, since the boards were all too big to fit inside the popper housing, and then there was the heat dissipation problem. The much discussed Harbor Freight Router Speed Control had all the qualities I needed. And it was on sale for $19.99 and with the 20% OFF coupon it cost just $17.40 with the tax. What's great about it is it is a self-contained unit so there's nothing to build. It worked actually better than I had anticipated. I had read in the beer-making forums that people had to add a massive heatsink to the aluminum back-plate of the case, some even going so far as to mill a depression in the plate to better fit the heatsink. The first time I used it, I set the VAR control on maximum until first crack, about two minutes. It did get hot enough that I could only touch the back-plate with two fingers for two seconds. When I turned it down so the beans were barely swirling around, it was warm, not hot. The next time I put the switch on FULL until first crack and then switched over to VAR and turned it down until the beans were just barely swirling. This time the back-plate was barely warm the whole duration. So great, no need for a heatsink. I have to say that, even though the control also reduces the fan speed, the roast is slowed down enough so that it is much more controllable and much more even. So this is already a huge improvement. Phase one completed.

Next I have to come up with a way to control the fan independently, using a minimalist, read that cheap and compact, approach. I'd prefer to have everything inside rather than have an external box with a separate wire. I'll report back on what I find.