We have done a couple avalanches, you can put them under the seat in a removable enclosure or we have rebuilt the center console to houe subs also.
center console w/ 2 10" subs
http://www.***boat.com/image_center/...re_040-med.jpg
box for 2 10" subs under back seat (we have done 2 12" subs)
http://www.***boat.com/image_center/...re_083-med.jpg