L7 15 ported box

Port displacement is everything you build into the box that comprises the port structure, including port airspace and additional walls.  A slotted port like the one you described here is given as W X H X L.  Width X height is the opening size of the slot, showing the amount of airspace opening of the port.  Length is how deep it extends into the box.

They have included the displacement of the sub, assuming it is mounted on a 3/4" front baffle.  If you build the front baffle in two layers or it is thicker MDF, the displacement will be less.  Additional displacements that you have to account for yourself include the port structure and all bracing you build into the box.  Except for round or cone-shaped displacements, all volumes can be calculated as W X H X L, giving you cubic inches.  There are 1728 cu in one cubic foot.

Slotted ports are usually built so that one wall of the port shares duty as a side wall of the box.  The port structure usually extends from the box top to bottom, so those port/box walls at the ends of the port are shared walls too.  That leaves one additional wall that you must add to separate the port from the box airspace.  When using 3/4" MDF for construction, add this 3/4" to the slotted port width to arrive at cubic displacement.

The info you detailed gives a slotted port opening of 3.5" X 16.25".  Calculate this opening area as square inches.  This is 57 sq inches of opening.  To find the same opening size on your box design, decide how you want to to situate the port.  If it will extend from top to bottom, along one side of the box, use the measurements of your design to find one length (or leg) of the port.  Divide into 57 to find the proper width of the other leg.  If your box airspace is 28.5" tall, divide 57 / 28.5 = 2.  Your equivalent port opening is 2" X 28.5" and still remains 13.75" deep into the box.

Port tuning is found by using the comparison of port volume to box volume.  Various calculators will give you this, but first get the port construction and displacements detailed on paper.  Draw your box construction with all measurements detailed.  Every piece of material you incorporate for bracing will have volume measureable in W X H X L, so add up all these cubic inches.  Your box interior volume has to be big enough to give you the volume shown in the manual, with all displacements included, in order to use the port size as given.

Here is a calculator you can work with:  http://www.carstereo.com/help2/Articles.cfm?id=31

If your net enclosure space is 6 cu ft, the port size you described would make the tuning frequency 34.25 Hz.  (With this calculator and the info that you know, try different tuning freqs until you arrive at the port length given.)

The port itself is a structure that takes up displacement, as I said above.  Find the displacement by calculating cubic inches and converting to cubic feet.  The only walls you have to consider for this are walls built for the port, not shared walls.  The dimensions you gave show a total volume, using 3/4" outside walls, of 6.8 cubic feet.  https://www.the12volt.com/caraudio/boxcalcs.asp gives you a volume calculator.  I prefer to use this calculator using only interior dimensions and then inputting '0' in the wall thickness box.