I had a heck of a time getting the steam box to heat up to 200 degrees plus. I eventually used a 30 quart turkey fryer pot with 2" pvc fitted to the top and to the side of the steam box. I had to add R13 insulation to the side of the box plus two layers on top before it would heat up and maintain 200 degrees plus for an hour.
When under full heat and the water is boiling and steaming, the pot will lose about 4" -5" of water per hour, so water needs to be added while you steam. After a bit of working with it, I determined that I needed to put in a picher of water every 20 minutes or so. I had a hole in the top of the lid where I could put a funnel and fill the pot as needed.
The little thermometers were placed near the ends of the steam box and they remained equal as the steam came in and raised the temperature.
One suggestion: Never use pine boards to build a steam box. After one heating up they warped all over the place. Steam leaks all over the place. Use quality exterior grade plywood at the minimumn. After messing with the wood box for two weeks, I think it would have been cheaper and easier to get thin wall steel tubing 8" x 8" or bigger and cut to the length you need. Add end caps and a steam pot and you would be ready to go. Insulate it if need be.
