The "horse hair" pads I think was an application from years and years ago.
The late 70's and early 80's cars had pads that were fiber in nature with a heavy application of a flexible glue or something. The later models had foam pads which were of rather high density.
I have tried the foam pad substitution route with not great results. The bolsters, top outside edges get crushed quickly as does the rear section. You would have to model new foam. BTW, these foam pads slip over the spring assembly. I put on a sheet of the camping mat you described but took it off again because the loss of the bolsters. I finally inserted between the springs and the old existing foam pad with medium results.
I am afraid the best is still a new pad. It is shaped and designed specifically for your seat.
As an aside, the covers hold the pad and all together and locate it all on the seat frame/spring assembly with considerable tension. This keeps the seat from sliding from side to side or front to back. This tension also makes the seating experience quite consistent.
As an old engineer once told me: You will need to adjust your seat position and your body from time to time in a good seat. In a bad seat you won't since it won't make any diffference. I have found this to be true.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)