would anyone on here be able to help me design a badpass enclosure for 4 18" BL's. The max dimensions are 21H x 49W x 72D and i wanted to tune it to around 29-30 hz. if someone can help please let me know asap.

anyone???? if not could someone direct me to a site or someone that could?

you on your own there boss...that's something that one of us would have to be there to do

I don't think you have enough room for a decent bandpass with those measurements and 4 18's..

I could do it. Your tuning frequency should be a lot higher though, like around 55 hz. I am thinking a -3db around 35 hz with about a 3db gain in the passband. I could actually make it smaller than your given dims too.