Extreme,
I built one out of plywood. It is 4' x4' x12' high. I have a cement floor and cement blocks on the bottom. It is double walled and insulated. I use propane for heat and put my wood chunks in an iron skillet on top of the burner. I made the racks out of 2x2's and 1/4 inch mesh wire. Each rack is approx. 2'x4.'' It works great. I can get about 25 big fish in there right now, more if I wanted to build more racks. I can regulate the temp right where I want it. I start it out at 150 and then turn it down to get a good smoke on the fish. I wouldn't do it any other way. It costs me about 3 bucks in propane for a batch of fish. The only thing that I have too do is move the racks as the fish on the outside gets done quicker that the ones in the middle with the cover I have over the middle. The fish in the middle was getting done way too quick. If you would like a picture of it I'll see what I can do.