Joe,
MB carpet is not cheap. I got some out of California for $55.00 a square yard. In fact, there's not much at all about these cars that are cheap. ha!
Here's a link to a speaker enclosure that's similiar to what I got off of this site. This one looks like it might tip over pretty easy since it's only 4" deep. MIne are more triangular with a wider base, but I couldn't find them on the site again. Mine are the same brand name as these so you might find others elsewhere by doing a different search. However, I highly recommend etronics for the speakers. Prices seem to be pretty good.
http://www.etronics.com/product.asp?stk_code=pyrpmb69mt