I'm with Matt, the foam board is cheap and if you can mark it close enough then it should be a nice press in fit. For the 15 or 20 bucks, I think I would try it first. Then once proven, I would make it a little more permanent. I also think that with just a little playing around with the fire, that you will have an awesome smoker.
Go Big or Go Home.....Thats what I say
.One of these days I want to learn to weld real good
.........Sometimes I feel like a nut.