L7 Port Info Needed

Your box as described looks to be close to 1 cubic foot smaller than the specified 6 cu ft box in the Kicker manual.  You must realize that the port dimensions are in relation to the box volume, so in order to use the specified port size (or equivalent) you have to be working with a box of the same interior cubic feet.

But then, that is just by doing a quick calculation using exterior dimensions that you posted here.  It's the interior NET volume that is important (net volume is just the air space dedicated to the woofers, taking out the port and all interior wood).

If you have arrived at exactly the same interior volume as the spec'd box, you can use the spec'd port size.  And you can use an equivalent of the spec'd port size...such as a different configuration of the port opening as long as it matches the same square inches.  And you would keep the port length the same as spec'd.  You would take their spec'd port opening area and find square inches (3.5 X 16.25 = 56.875 square inches).  Then you could change that opening to better fit your box so that it equals the same square inches, and leave the port length as specified.  But remember that the interior volume of 6 cu ft has to be there first.

If you are changing cubic feet volume from the Kicker specs, you have to calculate the port yourself.  Let the forum know what you are doing in this regard. 

Remember that the interior volume has to be large enough to compensate for the volume of the port and its walls, since that volume is encroaching in your woofer's air space.  IF you have close to 6 cubic feet after all dispacement allowances (note:  check on this...IIRC Kicker allows for the sub displacement with that number) then you are close enough just to modify dimensions of the port and leave the length as stated.

Very slight changes in box volume or port volume should result in only slight changes to the tuning frequency of the box and are usually acceptable.