A:AnswerI have a P3D2 mounted in this and it just barely touches the back. That lists as having a depth of 6.66 in. I would say you want to be under 6.25 inches. I added a 1/4 in gasket and there are no issues.
A:AnswerI have this box behind the seat of my 79 c10 with a skar audio shallow mount 12. Sounds great and fits good. Not too sure about the Rockford fosgate sub you want but it shows the mounting depth when you go to buy a woofer so you can compare them.