I don't really see why you'd need one with any kamado-style cooker, the vents work fine. Even my Akorn will hold temps like an oven for 10-12 hours with only very minor adjustments. To me, learning your pit and how to control it is part of the art of cooking bbq.
...Half a yard full of crap to cook on like everybody else...
Just a hungry hillbilly lookin for a dead critter to cook
in one throwdown,