Am I the first to post here? I am gearing up to build my own THB. My start point is this:
http://www.lulu.com/product/ebook/how-to-build-a-top-bar-hive/15321150

I am still pondering the a few questions:
Material: there is no 12x1 board, there is a 12x2, but that is really 11.5x1.5
Window: glass or plexi-glass
Entrance: in the middle or at (both) ends.

Jorg