man, that should be pretty fun when done.
i make my minis different, but your way is better. i just nail down a sheet of plastic to a board near the middle (depending on how long i want the flat to be), then nail blocks under the opposite sides of the plastic until the desired tranny. you have to be careful because if you have too much of a flat, and you want the tranny high, it's easy to snap some kinds of plastic.