download "winisd", google it
Calculate the box you want (put in desired net box volume, desired tuning frequency, and port area), it'll give you back port length.
Add up all the internal values (port VOLUME, port board VOLUME, desired net volume, sub displacement), and build your box to house that internal volume
tah dah!
it should be fairly simple as long as you can do basic algebra, since the only two shapes you will be using are squares and triangles to calculate volume.
1 cu ft = 1728 cu in, for calculation reference