Has anyone used PSI recones for the XXX V3? Can I recone from a 12 to an 18?

Been broke for awhile, figured I might as well use what I've got (05 XXX 12 & Visonik 1500W/both broken, cheaper to fix than buy new).

Can the 05 XXX 18 be used in a 4th order wall for 20-40+ Hz/daily?

For a 4th order can the ported part of the box be entirely open, like there's no port length besides what the sub diameter takes up?

For instance, with upfire, box sitting behind the front seats of my Alero, I can fit about 9 cubes sealed and 3-5 ported.

Sealed is 49 (door to door) x 20.5 (front seat height without head rest) x 20.5 (probably couldn't fit deeper anyways) = 9.3 NET.

The height of the car only allows for 39" width from ceiling to other side of ceiling (like door to door, just the top part, obviously it'd need to be slanted), which gives upwards of around 300" of port area, would that be enough?

Would the port more likely be the blue or red?

If red, I'd make the sealed part 39" instead of 49" top & bottom to avoid cutting angles (7 sealed), unless I'd need that space. An online calculator, for a 20 Hz cut off and a .5 response says 8.97 sealed & 3 ported, and it's tuned to 40.62 Hz. If it's tuned to 40 Hz yet says -3dB @ 20 hz than that's pretty flat, right? Suggested port length 8.5" and diameter 5.4", high-cut off 82.48 Hz.

How can it be -3db @ 20 & 82.5 Hz yet be "tuned" to 40.5 Hz?

http://www.colomar.com/cgi-bin/4thorder_proc