I’ve purchased a seat cover set for the rear seat of my '72 2+2. In contrast to the front seat, the foam for the rear seat is not available in molded form.
I purchased an interior kit for my Ser 1.5 2+2 from BAS UK. You are right, the rear seat foam was not molded. It was constructed from blocks of foam glued together to approximate the shape. I used a die grinder with a 2" 40 grit sanding disc to shape the foam to replicate the old molded part. Not perfect but pretty close. Picture is what I received. Cheers Richard