Thanks for resurecting an old post. Going through this brought back some good memories in the old shop.
For a nice, low velocity fan try to find an old desktop computer someone is throwing away. The box portion will have nice little cooling fans built in the back. They are small and will move just the amount of air you need in the box.
Time wise...a good weekend project...maybe 6-8 hours total. Cost wise, with the ETC this is probably a $150 project.
One other addition I made last year to this box was to wire in a timer switch prior to the ETC (I believe it is a 6 hour switch I picked up at a home improvement store for $17). Historically, I would either leave the oven on too long (i.e., overnight), have to time my bake right, or have to go back to the garage at an odd hour to shut it off. With the timer switch, you can throw in the item you are cooking....set it and forget it.