Box help fu 750v2 18

[Bryce1400]

I'm try to get the most output out of this sub. I like very low  bass like tech n9ne  Stevie stone, ritzz. Im running a 1200watt rms Rockford Amp and my space available is 38x48x20. I'd really like to go in the loudest/ lowest plan possible.  Any help or direction would be awesome