Port sizing

[TRON]

Im building a new box for my L3s (12 inch) and the manual says that the port should be:

Opening: 3" x 12.5" Length : 20.5"

The inner displacement of the port is 768.75 cu in.

My problem is that the dimensions of the box Im building wont allow a 20.5 inch port length. However, I can adjust the port dimensions to match the same inner displacement, but with a different opening size.

So, my question, is the port size that the manufacture recommends based on the displacement or the dimensions? Or does it not matter what the dimensions are, as long as the displacement is the same?

The box that I have currently calculated is tuned at 34.5 Hz with a total inner displacement (including driver and port) is 3552 cu in. The max for the driver is 3888 cu in. All of these calculations here are considering on side of the box, its a dual driver enclosure that is divided.