How to make port box 03 Silverado EXT help
 

Hey guys, got my system in the mail a couple days ago and while looking around I found some ported boxes but none of them would fit under the back row seat in my truck.
So I'm stuck having to make my own custom ported box (not that it is a problem fabrication wise) but I'm stuck on how to get the proper cubic feet and tuning frequency.
So.. To start I am installing a Rockford Fosgate Punch P2D4-12 in a 03 Silverado Extended Cab and want it in a ported box. Recommended cubic feet is 1.79, Vas is 81.5, and I'm finding some websites say Fs is 27.2 and some also say 40.
My problem is finding out how to make the box so 1. It fits under seat 2. It is close to recommended cubic feet 3. How to get the tuning frequency of the box correct. Am I suppost to make my own opening in the box or buy a vent tube??. If anyone knows where I can find dimensions for boxes that'll fit my truck, or knows websites that help with this stuff or anything it would be greatly appreciated. Thanks!