design sub enlosures for boat

I am new to this forum and was hoping I can get some help designing enclosures for my boat. I am currently using Kicker Comp 10's (in truck boxes) but I plan on replacing with CVR 10's or even 12's if I have enough room.

My plan is to replace the two boxes that the driver & passenger seat sit on with a custom built sub box's. I want them to fire forward

Dimensions of existing storage boxes are as follows:

Height - 13 1/2
width - 14
Length at top - 39"
Length at bottom - 31 1/2"
The seats overhang the boxes by about 2" on either side (width)

I should have enough room to use sealed boxes according to Kicker specs but I am wondering if I can get a ported design in this space. Being that my dimensions cant exceed 13.5" high and 16" (at the most) wide a typical ported design wont work. I am thinking maybe a Single Reflex Bandpass Enclosure might work. I also like this idea because it will protect the speaker from getting kicked & I would still like it to fire forward. something like the picture at bottom rotated left 90 deg.

I have no idea where to start on this type of box design & was hoping to get some help or input.

I am a cabinet builder so the actual construction is not an issue what I need help with is dimensions and all the technical stuff.

I can't imagine you're looking for a burp monster (bandpass) for your boat, so I'd say stick with the small box specs for sealed, and protect the driver from being gouged by adding a protective grille.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

Thanks stevdart, that's probably what I will end up doing if nothing else works. Let me ask this though; would it be possible to have the whole box about 4 or 5 inches off the floor and have both the driver and port face downward? this would give me roughly 39"(l) X 14"(w) X 9"(h). per driver. assuming this would work how would I design the port (PVC with 90deg fittings?)

There are two questions there, so I"ll start with number one:  yes, downfiring is fine.  There IS a sag factor requirement but it has seemed to be a rare case, in my experience.  That is, the weight of the driver is too great to withstand the sag factor of age, much like our jowls and other things.  ;)  But considering that it is okay for the task, over time, you just have to consider the square inches of room from floor to driver, times the four sides of the speaker box, to arrive at square inches of area that the speaker is operating into.  At 4 or 5 inches off the floor, you'll have greater opening area than you need.  You've probably noted that most store-bought floor-firing subs are much closer to the floor than that, so you'll be good.

The second question pertains to...the port?  A sealed box has no port.

If you decide to go vented, work out the design in WinISD or another box-building software program.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.