So I spent three hours working on the sled for the heater this morning. When it was all done, it looked pretty good. The frame was nice and square, the heating element securely held in the center, no gaps for air to leak around or through. Very nice. The only problem was that it didn't fit. Apparently, I got one of the measurements wrong. I know which one, and I'm kicking myself, but that does very little good.
They say measure twice and cut once, but it doesn't really matter how accurate your measurement is if you've measured the wrong thing. Tomorrow, I get to spend more time with sheet metal, getting frustrated all over again and adding to the collection of nicks and cuts on my hands. Joy!