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I installed a little "pusher" fan on my freezer conversion to facilitate a little more air circulation than convection alone provides. It seems to have helped equalize temps a bit. I used a used computer cooling fan.
Make sure you build plenty of thermal mass into the bottom part of your smoker. Like a heavy plate to absorb heat and also divert the air flow towards the walls. This will help equalize temps by forcing the hot air flow to expend energy heating the bottom of the smoker before rushing out the top. My freezer build is within 14 degrees (worst case) across the shelves, top to bottom. Everything cooks evenly. I made mine with a smaller freezer, chamber area is 20x22x40 with 5 racks. The taller you go the more difficult it may be to balance temps top to bottom, but you can tune it in.
Thanks guys for your input. I thought about rotating the racks, but would like it to where I could get everything inside done at approx the same.
I like the idea of the fan and have thought about that as being the best way to do it, but I was thinking about installing a used fan from a convection oven and actually installing that in the smoker. My only concern with that is the build up of the smokey glaze that you get normally on the inside of your smoker. I guess you could just clean it.
DaveNH, if you get a chance, shoot a pic or 2 of your rig my way, would like to see it.
If you get an old metal lined fridge with fiberglass insulation your already 3/4 done, check out craigslist for a vintage 1950's ~ 60's fridge. Insulation and liner is good to go. I like smoking with charcoal, but the added electric element is great for low temp smoking.