i don't know if you are still needing box help, but i came across this thread, and here is the specs straight out of the manual for my solo x12. it sounds you would need the tuner setup. port tuning depends on port dimensions. follow what is here.
Enclosures
S12X
Recommended Vented Enclosures
-Tuner : 3 cu.ft. Port: 45 sq.in. x 18.75”
Fb= 40 Hz, Pmax= 1400 W
( with 25 HZ subsonic filter)
-SUV: 6 cu.ft. Port: 60 sq.in. x 6”
Fb= 45 Hz, Pmax=2000 W
( with 25 HZ subsonic filter)
-SPL: 6 cu.ft. Port: 250 sq.in. x 9”
Fb= 68 Hz, Pmax= 2500
(Above tuning freq. only)
See below!
The SPL enclosure is intended for SPL ONLY!
It is designed to be used with test tones above 68 Hz!
Playing music with frequencies below 68 Hz will result in
very, very, very bad things happening!!
For more advice on box building see your local Kicker dealer
or the Choosing and Building Enclosures page in the Tech Tips &
FAQs section of the Support area on the Kicker website,
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