It is an old metal box. I intend to add some insulation, either Styrofoam or corrugated cardboard or plastic to keep the temperature more stable. At first I thought maybe I would mount it on the outside of the incubator but as I think more about it I believe it might work better on the inside as this might be easier to clean than metal walls.
I will revisit this when it is finished.
I now have 2 incubators the earlier one I built which goes up to about 30 degrees C and this one which goes up to at least 40 degrees.
I found this link to the pdf file for The Amateur Scientist by C. L. Stong. This is a classic with experiments in archaeology, biology, physics, electronics, etc. These are projects from Scientific American's The Amateur Scientist column.