well, if you make your own mounts, you can fit just about any sized speaker you want in there <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/biggrin.gif" border="0" valign="absmiddle" alt='
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That would be the easiest way to do it, I have seen pre-fabbed kits online that do the same thing, but making it out of MDF would probably be a better solution.