Great job! A bigger box makes the volume lost to all the caps moot as well.
I would want to use something like this:
http://parts.digikey.com/1/parts/1622692-holder-battery-1-cell-9v-bx0023.html
to preclude having to remove the woofer to change the battery. I've found that these will fit next to the binding posts on most cabinets with the rectangular cutout. I did have to mount through the back panel on my 4412A's, since the plastic plate is so small, but I seal the back of the battery box to make sure there were no leaks.