I've kept my rocket oven at 350F steadily for an hour or two. It's helped by the fact that I have a couple of pizza stones in there, so the mass helps hold the temperature steady.
I've found that one bigger piece of wood works better for this than lots of small pieces of wood (which tend to burn fast and hot).
I think you could build a rocket oven for this task, but I can't make any guarantees. If you had even more mass inside the oven space, it would take longer to heat up, but it would be easier to keep the temperature steady.