I have a four winns 298 Vista and I am replacing my entire system focusing mostly on the speakers outside of the cabin because I spend most of my time outside and dont need too much in the cabin. I have the 6 1/2's replaced but I want to add a sub. There is very little space for a box, but the boat has tons of storage space (lift up a seat cushion and open "box-like" space) like most boats. I was thinking about cutting a hole in the fiberglass on the side of one of the seats and installing the sub firing out towards the rest of the boat seating area. The storage area under the seat is pretty much sealed except for a small drain hole in the corner. If I stuffed it with some poly fill would it work ok? do I have to use polyfill? I was hoping not to to reduce mold growth. It would probably be more of a free air then an enclosed box. What type of sub should I use? I was hoping for the MM1040UM. I have included a picture, to the left under the seat is the location I was shooting for.