you dont "Design" A burp box. you throw money at wood cut it put it in. put it on the meter. repeat until you peak. usually 4-5 boxes honestly.
the only thing anyone will ever know about "Designing" An SPL Box. is
Small box (Im talking .5-.7 for a 12) With a ton of port. I usually toss things around.
I start with a larger box and start shrinking it with 2x4's inside of the box glassed. Then for port erm.....well...bout the same on that too. (For me) Some do it different.