I got 22 liters by actually filling it up with water. I have a blendtech that has a 1 liter mark on it that was easy to read. I did one fine tune measurement to make sure it was correct and filled the whole thing up with water. until it started pouring out. Now this couldnt quite get some of the overhead from where the speakerframe was at so I was saying approximately.
That's actually how I thought you probably did it, which is probably the best way to do so. So, with that, I have another consideration:
Alpine SWR-8D2
This Alpine is probably a better subwoofer than the Kicker, but not necessarily in your application. I ran some numbers through two box building programs and came out with an enclosed F3 (so lowest usable frequency) of 53Hz for the Kicker, and 57Hz for the Alpine. So, if you seal that box the Kicker will go slightly lower than the Alpine, but I can't say which one would compliment the overall system better. I believe the Alpine will most likely be more "musical."
A problem I ran into is that Kicker doesn't release all the Specs necessary to fine-tune a box, so that 53Hz was arrived at with limited information. Alpine releases all the information on the woofer, so I'm pretty confident in the 57Hz.
As to trying to make a vented design, it would be difficult. Both the diameter of of the port and the length would have to be modified...and with the box configuration it is basically impossible to have a port long enough. I believe the suggested smallest matched size was 3.2" x 25.25"...which obviously wouldn't work.