Most old school Merc's simply modified their door sidings to accommodate speakers.
When I was designing for a friend's Merc (
https://www.facebook.com/photo.php?f...type=1&theater). I limited the "box" to around 1 cf sealed and it was a pain combing through files of midbass drivers that's available locally that is suitable for the box. I believe I settled on Dominations D8s (8" midbass) on the doors while the midrange and highs were handled by the Dominations F50 (2" widebander).
Okay, kung di mo ma-gets what I'm typing, it's cool.
Just go to your installer and ask what your options are. However, you have quite a problem because you're pretty limited sa midbass ddrivers mo. You can't simply buy any run of the mill 5,000php speaker tpos salpakin mo lang siya sa DIY "box" because most midbass nowadays are designed for door applications/IB (salpak lang sila usually). So if you put speakers meant for doors in a box, you get two things: (1) it won't go as low and (2) it'll have a peaky response somewhere.