WHat about grabbing one of the Corbeau rear seat kits, and then cutting out the center material and having it sewn into the covers and then installed on your current rear seat?
https://lmr.com/item/CS-FB26501CV/83-93-Mustang-Convertible-Black-Cloth-Rear-Seat-Upholstery
+ your covers?
Ultimately it's what i'll probably end up doing. I'll buy another complete rear seat so a shop can use the original material as a template and then they can create the rear seat out of materials that will match the front, but the patten will be a Fox seat that I can install like OEM.
Of course that's down the road. Upholstery work isn't cheap which is why i'm happy with the Corbeau setup in the back for now