^Agreed on JL. I'm running a 12w6 which certainly isn't from their shallow-mount line!
Chameleon, the sub box I'm talking about went where the storage space is on the driver's side. You'd definitely want some protection against loose tools sliding, but it didn't encroach much into the trunk.
As far as the crown vic rattle...well, it might be better than the bowel rattle you might be experiencing with a shallow mount!

Seriously, I try to find that balance of sounding good inside without buzzing my trunk lid. You can fix that with dampening but I don't want to add the weight (the sub box is easily removed if I need to drop the weight).
In any event, good luck on getting your setup going!