help with comp box for 18"

Hey all I got my self a crossfire xs v.2 D1 18" subwoofer my current daily box I fogot about its low tuning (32hz with 7.5 cubes) my port is 7" wide and around 200sq in ir more. I took it to state finals yesterday and hit a 143 on a sweep and 142.7 on on music came in 1st place stock suv 2 and 3rd in featherweight. My resonance frequenc was 38hz so here we go if I build a box out of birch how much space should I gave the sub and then what should my tuning be to peak at 38hz? Port size everything I'm trying to get around 145- 147 so I can keep up with everyone in featherweight. Thanks all!!