I have a 2001 GMC SIerra Z71 extended cab truck.
I have built 2 sealed boxes, 1 each, for 2 CDT MS-80 8" SQ subs. The boxes are .42 cu/ft and the spec is for between .25 and .45 cu/ft.
The subs will be going under the rear seat of the truck. I will have between 1 and 1.5 inches of clearance...