messure the back of your cab from the flip out seats across and from the rear wall to your front seat bottoms (straight across do not include the drop down. cut a piece of plywood to the size you came up with. than cut 2x4s for it. if you cut 2 6 in piece you can make a cubby hole in the middle. attach the 2x4s to the plywood carpet the top and front sides.. this will give you a good flat area in the back to set your big sub box and amp.