Box is for a 12" Fi BTL (loaded)
31(w)x16(h)x18(d) are the MAXIMUM dimensions and would like to use all of it.

I used subwoofertools.com to design my last box for a 12" Fi BL @ 32Hz.
But, that website is down, so looking for alternatives.
I've tried a few others: WinISD, carstereo.com and they have all yielded different results.

My new box I would like tuned to 31Hz.
You'll see in the picture that it is lower, but to account for displacement I figured once the woofer is installed it would bring me to around 31Hz.

It just seems that there is no official way to design a box, unless there is something I need to know about?