I had an 81 MC back then with four 6x9s on the rear deck. I made a custom plywood rear deck and covered it in cloth. I cut it in half so I could get a tight contoured fit end to end, but covered both halves in one piece of cloth so you couldn't see the split/cut. It looked pretty good to me.
I don't have a picture of it, but I do have a picture of a picture of the three piece rear deck I built back in 1986 for the SS. On the two far end pieces (covered in a linen material) I had 5.25" Acoustic Research plates and Concord Parabolic Coned Coaxial speakers mounted up top. The center section (lined up with the seat's seams) was covered in speaker cloth with two "free-air" 10" subs mounted under the rear deck.