Ours are called "Phoenix Dry Rod" ovens, but I have just recently heard of one called "Hot Box" http://www.mathey.com/181.htm , might give em a call. You can make one, just make sure it will perform the duties of production made oven. Have to have it vented to let out the moisture that is trying to get out at 250 degrees F. My opinion would be to buy one already designed and ready to go to work.
John Wright

I started out in the structural steel fabrication business in 1976, and at that time the shop I worked in had a couple of home made ovens. They had made two boxes out of 1/8" thick steel that looked like they could hold a couple 50# boxes of rods. These "ovens" had one shelf each made from expanded metal, lined with tin foil and had a 100 watt light bulb in them. I never knew whether or not they reached the 250 degree minimum, but the rods were warm. Four years later when we applied for Category II Certification, the ovens were quickly converted to keeping food warm, and the guys could keep their food in them all day long. When the Company purchased rod ovens for the audit, the first day they were plugged in one of the guys took his small can of beef stew out of the old oven and put it in the new oven to keep it warm. At last break he opened the door and found that the can had exploded. I guess the old oven wasn't anywhere near 250 degrees.