If your xmax is accurate then a 5 cuft box with 20hz tuning and a 6" square port that goes the length of 43" will work if you plan to use a 4th order L-R filter at 20hz.
If you prefer to use a 2nd order butterworth filter you'll need to step up to 7" x 7" x 60"
I will say if extension isn't your goal that you can certainly tune higher. I designed this so there is only a 1 db drop from 100hz to as far down as the xmax would allow in an unfiltered design. If you want to go with a bigger box to really stretch the sub. The furthest I'd go is 7 cuft.
You can adjust the square to a rectangle Just make sure to keep the ratio as close to square as you can.
You will want to place a large section of bracing behind the driver and cover it with 4" of 8lb rockwool to eliminate mechanical noise. You can add additional rock wool to cover have the internal surface. This is up to you don't overdampen it. See the Infinity Kappa perfect thread for ideas on how to brace and dampen properly.
Make sure you wrap the rockwool in fabric.
This sub can deliver 120 db at 20 hz with room gain that could be insanely high.