Came across this on the tap plastics website today. 1/2" acrylic would make a decent medium sized speaker box. with a brace or two:
<A HREF="http://www.tapplastics.com/info/vide...rmat=quicktime">http://www.tapplastics.com/info/vide...rmat=quicktime</A>&

Here's one on cutting plastic:
<A HREF="http://www.tapplastics.com/info/vide...rmat=quicktime">http://www.tapplastics.com/info/vide...rmat=quicktime</A>&

how to glue larger pieces:
<A HREF="http://www.tapplastics.com/info/vide...rmat=quicktime">http://www.tapplastics.com/info/vide...rmat=quicktime</A>&