need help....ported enclosure...sparq?

gee havn't been here in a while. i am building a new enclosure for a friend of mine. it is housing JL audio 15" W-3 D4's..the enclosure will be split into 2 boxes. the plans for the "db monster" enclosure call for 3.25 cu. ft. air space with a 5" dia. x 14.9" port. how do i go about converting this round port dimension into a rectangular port dimension. these are going in a nissan frontier king cab so the box dimensions aren't gonna work and you can't get 5" pvc. so i've decided to go with rectangular/duct ports. if anyone could help it'd be greatly aappreciated

buy some fiberglass

yae here's a good site for box building
http://www.eatel.net/~amptech/elecdisc/spboxnew2.htm

go about half way down and you will everything you need in the calc.
What is that port tuned to?

the port is tuned to 29.6 Hz