Free-air sub for our boat.

I am getting close to having everything for our install that hopefully will be done this week.

Right now, the system consists of the following.

6 Pold DB651 speakers in the boat powered by a JL Audio 600/6

4 Polk DB-651 on the tower and 2 Kicker KMT6 on the tower powered by a JL Audio HD 600/4

The HU is a Pioneer DEH-P5000

I already have an Alpine PDX M600 to power the sub(s).

Originally I was going to go with a couple of Kicker SKM10 as I can get them off of ebay for about $165 each. But, they would be mounted in enclosed storage compartments and that concerns me. I'm afraid it will baffle too much of the sound since there is not an opening and for the sound to get out.

I didn't want to cut more holes in the boat, but I am starting to lean that direction.

From an amp standpoint, if I needed to I would not be too opposed to buying another M6 or an M12.

At my disposal, I also have a Kicker KX 850.4 or a Kicker KX 650.4.

I am not 100% opposed to building a box to fit, but I would really rather not.

We listen mostly to country and rock, some rap, some R&B, etc.

I would appreciate any help or guidance. I know there are a lot of varying opinions and I am not really knowledgeable on subs. Budget is a consideration, but I have spent about $1500 already so what's another couple hundred.