I'd go with the recommended boxes, there's not much to fiddle around outside the recommended designs.
• Sealed (home theater/music): 4 cubic ft. (net internal, not including driver) for an f3 of 31 Hz, f10 of 18 Hz with a Qtc of 0.72
• Sealed (music): 7 cubic ft. (net internal, not including driver) for an f3 of 31 Hz, f10 of 15.8 Hz with a Qtc of 0.62
• Vented (home theater): 8 cubic ft. (net internal, not including driver or vent) tuned to 20 Hz, with 3 flared vents 4" diameter by 28.5" long, for an f3 of 18 Hz
• Vented (infra-sub): 12 cubic ft. (net internal, not including driver or vent) tuned to 15.5 Hz, with 3 flared vents 4" diameter by 32" long, for an f3 of 15 Hz
If you got enough power you could also use a linkwitz transform to make the low end response better in a sealed alignment.