custom box build

[CookiesCustoms]

I building a downfiring box for one soundstream trantula 15". Im doing the port to run up from the inside back of the box. Im rounding the port due to me rounding the top of the box and im wondering if i should keep the port diameter the same or should I start with the port diameter and increase the size to the opening to kind of favor a clam shell design. Im wanting to keep the box tuned around 32hz and 4 cu. ft