I also built a 6x9 box like the one from msa.  and it only costed me, get this, about 5 bucks to make.  I just went to home depot and bought some hard board for about $5, and then used poster board to make a template, and did some cutting with my table saw.  Then I used some smalll nails I had to put the box together.  It looks much nicer then my old ripped up vinyl panel.  I havnt installed it because I still need to get an amp for my 6x9s.  I also need to cover it with fabric.  I may still decide to but my 6x9s behind the seats of my car