Here is the box that I made. I used a tongue and groove that ran the length of the box to seal the sides and Gorilla glue to seal the joint. Since this photo, I've added a proper hasp.
The insulation is 1 inch thick foil faced and is just draped over the box. Without the foil face, the insulation will swell and warp. With this set up, and insulation on both ends, I was able to get the box up to 210.5 degrees f. With 212 degrees being the absolute highest that the temp can get without being pressurized, I call this a success.
When we opened and closed the door to retrieve the bends, the temp only dropped about 6 degrees and recouped the heat within minutes. One thing that I noticed was that one of the two steamer units went through water much faster than the other, and so I checked the wattage. Sure enough, my older one was 1250 and the new one that I bought at Highland was 1500. While the bends came off beautifully, I can only imagine that two 1500 watt units would be the ideal. Keep in mind that this box is 9 by 11 inches so that it can hold a whole class worth of parts and must keep the heat up with lots of door openings. On a smaller box, one unit is fine, especially if it's insulated.