Just wanted to chime in and say what a great job on the kiln!
If you're drying wood mostly for your own use, a modification to consider down the road would be to add black metal roofing panels to the underside of the rafters creating a pocket between the glazing and the roof panel. By building a box for the fans that directs airflow through the pockets, out at the bottom, and through the pile, the kiln can be "turned off" simply by unplugging the fans. This is useful to use the kiln as a dry shed for lumber storage when not kiln drying (which is what I do). Temperature increase with fans off is 5-10 degrees above ambient in the summer. With fans on, temp increase is up to 45 degrees in my kiln (140F is the top temp I've recorded - I think the triple wall polycarbonate panels help a little). The kiln modification is a lot of extra work but I dry mostly for my own use and having an extra insulated, dry storage space while I work through my inventory has been a big plus.
Don't ask me how I know that!