Okay I am going to do a build in my single cab '03 Sierra in between the seats. The seats are 40-20-40 so the space in between each bucket seat is 11.9" The box will be 11.5" wide.
You figure that by taking a measuring tape and measuring it from the edge of one seat to the other, how close to the front of the cab from the rear wall you want to go, and how high you want the box to be.
Being only a single 10" you will probably have a much easier time not taking out the seats to put in a box as your center console and just stick it behind the seats.