Anyone have any idea how to do a down firing sub enclosure? I'm thinking of adding a second sub but in a closed compartment so I thought down firing would be best. Am I wrong?

I've had decent success with dowfire boxes in my last two boats. They are relatively easy to build. leave your self around 2 inches of downfire space.

you will get more sound if it faces the hull.

Down firing works very well. Very musical. The size of sub dictates the size of the enclosure. The manuel will give you the proper size. IMO facing the sub towards the floor produces a richer base because the hull resonates.

2nd the hull facing if possible