I currently have 3 Soundstream Picasso 12"s on a box I built myself a year ago (5.5cf 32hz 52sqin port) and have been told by many my port area is ridiculously small which I agree with. Rather than just build a new box for them I've decided to upgrade to an XFL 15". My current thoughts now are 4 cubes tuned between 30-32hz with 64 square inches of port.
What I don't like about my current setup is that the subs face the back and are pretty much on display as it's a hatchback. So I'm hoping to go with a downfiring setup, but while I'm at it go with a horn type design. I've done calculations to see if the sub is suitable for a downfiring setup and with such a stiff suspension it's fine. This is a general idea of what I'd like to do.
Would this be very hard to do? I'm hoping to have around 4" breathing space for the port at the minimum then go increasing in size. I'm really just after thoughts on this design and any tips I should take into account for example the angle of the horn coming up. Which btw the larger end will be facing the driver so the port exit will be at the far back of the car.