Drums are hard to weld without warping, and burning holes in, especially if you plan on using 1/4 inch. They are also very hard to get good seals on because they become flexible when cut, don't get me wrong all this can be done, just much harder. You may want to take a look at some heaver metal for the cook chamber, in the long run you will be happier and may not even need to insulate it. A round cooker is going to be a bit hard to insulate, just my two cents but I would like very much to see the end results.
Let us have a drink and by God lets us not think about the things we ain't never going to know about.