Hows about modifying a house floor vent & operate it with a manual choke cable?
Funny, as soon as I read the post I thought the same thing. Air vents some in many sizes and shapes, if he got one and redid it so the adjuster is on the backside and filled in the hole it would look like a custom grill. He could even hook it up with an electric motor and a temp probe to automatically open and close the vents as necessary. I can't do it, but I imagine any electronics tech who might be posting here could design a small circuit board to open and close the motor depending on what voltage comes across from the temp sensor. Wouldn't be too hard to build on a breadboard if the design were worked out either.