Need assistance with new sub enclosure

[all4spl]

Ok to make a long story short I got a new vehicle last sept, 2011 Jeep Liberty. And prior to that I had a car in which I built 7 enclosures to find the MAGIC one that gave me exactly what I wanted. This process is one I hope to NOT repeat with the jeep, however box #1 was not even close to what I wanted. I built it based on many recommendations to repeat my old CRX style box of subs up and port back, in which I did.

The sub-woofers are Sundown ZV312's dual 1ohm, ran on a Skar SK4500.1 with 3 XS Power D5100 batteries. The first enclosure came out to these specs; 15" H x 41" W x 18" D. Yielding 3.46cuft NET tuned at just below 35Hz, the port is 13.5" H x 4" W x 26.75" L with 1 extra baffle and accounting for the subs .42cuft of displacement with 3 common port walls . The box plays well, however the lower end from 35Hz down just isn't there, and while I know playing below tuning isn't going to be the last car I had would to it VERY VERY well with the same tuning but a side firing port in the back of a hatchback. So I "think" this time the vehicle is reacting differently.

I was planning on building a new box with these parameter's but I wanted to ask for some input from others who may be able to give me some tips or advice to get a bit more low end out of the jeep. My external dimensions are as follows with (2) of them being constant as I cannot use anymore space. 17" H x 41" W x 18" D the dimensions in bold are the ones that cannot change, so I can make the box HIGHER but I do not want to block my view out the back window (15" is just below the seat line as a reference). I spec'd the port for those dimensions at 15.5" H x 4" W x 32" L for a tuning of 32.33Hz.

So what I am looking for is around 30Hz to be pretty spot on, I know the subs can handle it, I know the amp has the power to do it, but getting the enclosure just right is what is keeping me from proceeding to build and build and build till I get the right box. Hopefully someone can offer some advice since I have been looking ALL over the internet trying to find people with liberty's and there are not many with systems out there that I can find.

Thanks for any help and I hope I provided enough information.