how it's DONE!

i need somone to tell me how it's done i want to fit at least one 15 in my b2200 and yea it's a standard cab, i took the measurements and i have an area that's 51 inches wide and 21 inches tall and 7.5 inches deep at the most i would love to get it down to 6.75 ot 7 inches deep how can i fit a 15 in there?