My oven is small, 30" diameter. There is a guy over at fornobravo with a 21" oven. Small ovens lead to smaller fires, but there is also less to heat. So long as there is room for wood and a opening big enough for sufficient airflow and oven should generate flames. Problem A1 here looks to be the vent. I can't really make it out in the picture but it look to be at the back right above the fire. This is all wrong. It looks like you twisted the barrel to get the vent height lower then the peak of the dome, but it still needs to be at the front. If you can get the vent worked out so the heat flow is correct I'd then parge the dome on the outside with homebrew mortar and cover that it lathe so when the mortar cracks it will stay fairly in place. That would be a cheap easy way to add mass that seems fitting with the building techniques used on the rest of the oven. Your probably never going to get this oven to hold 1000F hearth temperatures for hours on end like a traditionally build oven, but I'd bet with some tinkering you can get it to 700 or 800 for long enough to cook at least a few pies.