So like a normal every-day 15hz ported box then?
Probably best to make a 15hz ported box that is designed
for those woofers, rather than designed for different
The net volume, port length/size and port velocity might have different requirements.
The the width and height too, since this is a 21, not an 18...
You should start by using winISD to plug all the values in for the driver, the tuning, the power, and then optimize for excursion/spl.
Once you have a rough idea of the size, then you can use sketchup to mock up a box in 3 dimensions.
You can use the ruler-tool to figure out the cut-list.
This is what I did for the HzHorn, but using HornResp instead of winISD.