Here's what I made up for a center channel speaker.
http://cerchaos.homeip.net:1280/woodworking5.html

Its doesn't go super deep, but for this center, it doesn't have to.
The Fostex are shielded [sp?], and the pieso doesn't have a magnet
at all. If you have a CRT TV, you'll have to worry about this.
If not, you don't. Its sounds rather nice to me. The drivers
were less than $100 total.

If you use a real horn, Wayne's crossovers have a padding circuit
to get the output levels in line with the low end. Of the standard
Pi drivers, the 10" Eminence Alpha's a really nice, but you'll
have to sheild them somehow (maybe), and the box is going to be 12"
high or so.

Chris