[chromeitout]

Galvanized will be fine. 20 ga. minimum. 18 would be better. You will need a vent if the oven is sealed good, but it won't control temperature evenness, it's just to give the expanding hot air somewhere to go and also exhaust the fumes (best to pipe outside). I've seen some people on here put just a fan in for circulation, but I don't know how it worked for them. I actually built an exterior ducting system that takes hot air from the top of the oven and pipes it back into the bottom (of course mine's a bit bigger than yours). Anyway, if you don't circulate the air somehow, the top of your oven will be much hotter than the bottom.