I wonder if what you saw was my video series on this:
Mercedes W123 seat rebuild - YouTube

I replaced toasted W123 seat pads with memory foam. I got a gently used king size 4" memory foam mattress topper for free off of Craigslist. I used a cheap electric carving knife to cut the pads.

The seats aren't yet installed in the car, but I did sit on them with the seats on the ground. I'm happy with the way they turned out and can update this thread once they're installed and in use.