Alpine Type R Ported Box

Alpine Type R 12" (Dual 4ohm) The biggest dimensions I can fit in the space is 18" x 9" x 26". This is going in a boat, so I'm not sure what frequency to tune it to. I'm assuming a ported enclosure is the best option for a boat install? And I read that somewhere around 35-40hz would be optimal? I used the RE Box Calculator, and it gave me a design, but I'm not sure where to cut in the subwoofer. Do I have to have an "L" shaped port? I also looked at the Digital Designs box, but it's a little to big. It has a straight port. Any suggestions or links to readings would be appreciated. Would a sealed box be out of the question?

Well, I created my own with the help of the RE Box calculator. This is where I am currently Just trying to figure out if I want to invert the sub or not. The black box was the original box. It was roughly .91 cubic feet with 5 3/4" holes drilled in it, and a gaping hole where the cables came through the box. It was horrible, but hopefully I'll get it straightened out!