maybe you should just get some really beefy 10's or 12's and seal it off from the trunk with some decent power going to them
i'd imagine you'd have more than 8 inches of depth unless you have a smart car or a 2 door without back seats.
IMO, 4 10's aren't worth walling.
don't get your pants in a bunch, please. just trying to help you sort things out.
lets see some pics of your car