I use Aubers. There is a fan that fits in one of the holes in a bottom vent. All others are shut. You have a temperature probe that goes on the cooking grate and is connected to the contoller. The controller is set at whatever temperature you want to cook at and when the temp drops below your target temperature, the fan kicks on and blows on the coals till the temp comes up.
Other controllers have a lot more features, wi-fi, charts, graphs, etc., but I just wanted one to contol the temp and the Auber works great for that. I have both size fans so if I want to cook at 300 or higher I just put on the bigger fan and may have to play with the bottom vents. On a WSM wind is a big factor, probably more than ambient temp.
Smooth Smoke BBQ Team, KCBS CBJ, porch mounted Jambo.