I got JBL GTO 1514 single voice coil 4 ohms. 350WRMS, 15 inch. It came with manual and says

Vbox 3.50 ft3 Fb 30.0Hz, so is this box tuned to 30hz ? Im thinking of making new box and i really need ideas how to improve my SPL. I want to try different box, something like this:

At the moment i just have vented box with round pipe. but i want to make something like in the picture, so how can i calculate box size ?

Box size is internal H x W x L. Minus port volume and sub displacement.