6 cubes would be bare minimum. Personally, Id put them in closer to 8 cubes each. RE use to like to say 6 cubes because it made the driver look more appealing to a wider variety of people.
Ya, I'm doing 2 18's for my daily off a sundown saz3000d. It's going in the back of my suv, dimensions are 48W x 19-22H x 46D (That is right up to the tailgate in my suv so prob will do it a few inches from that for port)