New to spl looking for a good box design

[Jerid]

Hello I'm new to to forum and just getting into spl. I have most of my system already built but the current box just isn't cutting it. I'm running a powermaxx 250 amp alt, big 3 , 4 re sx pro 12s. Dual 2 ohm and an american bas vfl series 450.1 amp in a 2008 chevy avalanche. When I built the box I didn't know much about tuning ports so i just put some in. If anyone has any suggestions on box designs I would greatly appreciate it I am limited to a 49 inch width and a 18 inch height but as the subs are facing the cab in the bed of the truck length is not an issue. Also I've been told I want to aim for around 40 hz does this seem like a good number. Thank you