I had the same issue with a W115 and after a lot of searching found there was no good alternative to the MB horsehair. I obtained an OEM bottom and took it to an upholsterer who installed it with reasonable cost. The horsehair in the new bottom felt "rubberized" and looked like it would hold up very well. Made me wonder why I had not done it a lot sooner.

Like the rest of you, my first thought was a dense foam, such as that used in motorcycle saddles. I was talked out of it by the upholsterer who pointed out that ones weight is concentrated in a cycle saddle application whereas weight is distributed more in a car seat. The result would be that the dense foam would seem much harder.

My advice is go with the OE equivalent. The Benz designers know what they are doing.