There's been a lengthy discussion on enclosure types and sizes on the Digitrax Sound Yahoo group recently. Basically, there is no one right answer. The size, the shape, and the material used all have an effect on the sound, and by varying these you can customize the sound to a great extent.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Here is a link to a video about using sheet lead as a speaker enclosure.
This fellow has some more examples of different speakers if you know how to search the You Tube videos. He is a contributor to the Yahoo SoundTraxx group.
Yes, I know all about the potential health issues about handling lead and having it around the house where there are young children.
I use sheet styrene for enclosures.
http://www.youtube.com/watch?v=6zfuBNJyvlM
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.