I had a local HVAC dealer make me a sheet metal box 12x12x40, as well as a baffle 4" tall to place in the bottom.
I then put two electric charcoal starters in it wired up to a ceiling light dimmer switch. Drilled a hole in the middle of the box cover and put a candy thermometer in the hole.

To adjust temp, just turn the dimmer switch up or down till it reaches desired temp. Then mark the switch.

You could build the box out of 1x12's or 1x6's and line it with foil faced fiberglass insulation.

Sorry, no pics of it on hand and it's fouteen miles to town where my box is at.

"There are three kinds of men
Those who learn by reading
The few who learn by observing
The rest have to pee on the
electric fence and find out
for themselves."