box specs and a couple questions

This is from the image dynamics site and is specs for a ported enclosure for an idq-10.                       large       normal          small Ported Volumes      1.0cuft      1.0cuft      .75 cuft Ported Frequency    30Hz       30Hz              34Hz Port Dimensions 3.0"IDx15"      3.0"IDx15"    2.5"IDx13" -3dB In car\Free space              18Hz/28Hz      18Hz/28Hz      22Hz/35Hz Questions: I am a newb, so please forgive my ignorance. Do these stats tell me that the ported frequency will be the stated number based on the length of the port, or that this is what the ports need to be tuned to? Does it matter if the port is inside or outside of the box, or a combination of the two? What are ports made of? pvc? I am trying to see if I could design and build a box that would have a port that extends up through my rear deck....installer quoted me $400 to design and build one and I think that is way too much money. Thanks if anyone can offer any insights for me.

Thanks for the response...I really appreciate it. Once my amps get installed I will see if i can use some of the shops pre-built sub enclosures to test and see if bass can get into the cabin. Believe me, I would much rather use a sealed enclosure for both space and SQ, but everyone on the mercedes forum I belong to, and the installer who is a mercedes "expert" says there is no way that the current opening that houses a first-aid box (which will be removed) will provide an adequate opening for bass to enter the cabin. I also do not want to disrupt the structural integrity of the vehicle by cutting the metal that lies below the rear shelf to any great degree. There are 6x9's in the rear shelf, but they are housed between the rear shelf and the solid metal, so there really are no openings that could be exploited with their removal. I guess I really need to just listen for myself once things get installed. Thanks again....Randy