See pics of these that I've built at
Tonal Eclipse Mastering Studios on MySpace Music - Free Streaming MP3s, Pictures & Music Videos in the pics section. While there are no constants, 12 inch drivers work best for me...as do 8 ohm jobs. There's a fine line between having too much mass and a slow response and having too small a diaphragm and getting thwacky sounds. I house these in drum shells that can be tuned like a resonant chamber.