I looked at his presentation, and he says just cutting a hole in the tin lid or bottom even lined with the wool, the intense fire would just burn away the tin. If one is only dedicated to using small steel stock pieces, one size hole would be okay, but with the fire bricks, one can modify the size opening to what you are working on. Of course the addition of a door on the back would let you heat the middle of longer stock. Using the tin lid, even insulated, would not be long lasting, with the 2,000 degree heat boiling out of that opening. Also accidents happen and if you drop something inside, a couple of bricks can be pulled away to retrieve the lost piece. Do not have a gas forge but worked around quite a few at Hammer-Ins. They work well, and heat quickly, but for me are very noisy.