I mentioned in my original blog entry about the Herb and Seed Cleaning that I wasn't satisfied with the way I had used a cardboard box to act as the catch basin and hold the levels in place. It worked, but it was a clunky hack. I had a large sheet of Masonite lying around, so I used some of it to make a base for the unit and as side-panels to hold the levels together. It still ain't headed for the Museum of Modern Art, but it is now a much slicker, cleaner implementation, and stores more easily as well.
For the base, I simply cut a rectangle of the appropriate size and nailed it on with 5/8" bright wire nails. The pieces of Masonite that act as guides/braces for the sides are just 3"x3" squares screwed on with a variety of small screws from the big can-o'-miscellaneous-screws.
If you have any more improvements to suggest, please let me know!
UPDATE: November 2013:
I decided to an additional top level to my herb and seed cleaner. This new level uses chicken-wire as its screen. There were several cases where I wished that I had a very gross first filter; notably peas and beans, and also seeds like basil where the seedpods remain attached to a long flower stalk. My new chicken-wire layer catches many of the large pea and bean pods, and the dried flower stalks of basil, dill, and other plants. A fair amount of unwanted material does fall through the large gauge of the chicken-wire, but it catches enough big pieces to make further processing at the 1/4" hardware-cloth level much easier.