Read the following article: Testing An Open Bottom Board, page 37 of the January 2008 Bee Culture. You'll have a much better understanding of why I perfer SBB and slatted racks and top entrances. No need for closing the bottom even in winter if the top is vented.
It also refers to the "dead air layer" at the bottom of the hive. It's nice to see some of one's own discoveries verified, enough if not exactly in the same configuration. On the "Build it" inset I would just make a 1 1/2 inch notch all the way across the 4X4 and run the cross bar all the way across--simpler, easier to build, and sturdier to boot. Scientist, like Engineers, have a tendency to make things more difficult than they need to be.