Sounds like we're trying to reinvent the wheel somewhat. The simple design on bringing the stack down into the burn chamber, allowing the gasses to burn above it is one of the reasons I went with a Shaver. Well, that and the price. I know I've been critical of these things but overall, I'm satisfied. When I open the door during a burn and see the top half of the chamber full of blue flame, I know things are working properly. A square firebox, even without ripples, obviously has more surface area. It also isn't as strong and has more welds to fail. If you're getting flames out the stack, close the inlet door on the fan a little. I would bet that there were flames exiting a free standing stove with the flue damper closed, you just couldn't see them. Choking down a stack would be the same as having one half plugged with creasote, well documented in this forum that performance suffers. In my opinion an auto damper is necessary for decent effeciency out of a Shaver. With the fan shut off and the door closed. I can hold my bare hand over the stack. It can't be over 100 deg. The stack itself is a little warmer, I can only keep my hand on it for a second or two. Point is, very little heat loss there. I'm not saying these are the best boilers available. They are what they are - simple, functional, affordable ......WOW, I think I just came up with a slogan