I built my hot box like the one suggested in Dean Torges' book "Hunting the Osage Bow". Mine is 16" square and 7'-0" long, made of 3/8" OSB and lined with 3/4" rigid foam insulation. It works great.
Like Pat B, I used three ceramic light fixtures mounted on a board that can be removed. I use two wall paper hanging trays filled with water to regulate the MC. A $10.00 hygrometer from Lowes lets me know the temperature and relative humidity. A shelf the length of the hot box provides support for multiple staves or billets. The sides of the shelf have a 3/4" gap the full length to allow for good air and heat flow.
It's simple but effective and I would build another one just like it if needed.