Cheapest and easiest way to slow down a motor

[Gdeaver]

You already have a great DC 12 volt speed control in your possession. A cordless drill has a pretty good electronic speed controller. However, the problem is you want to run the motor continuously for many hour. It may be better to let the motor run at full speed and use belts to speed reduce. A 1.5" pulley and a 3.5" adjustable pulley would give a good range. A couple of shaft inserts would get you from a .5 shaft down to the van door motor shaft.
To bad you don't live in the frigid parts of the country. Heat build up wouldn't be a problem.