We have all read about the foam “pool noodles” to firm up the seats - and the noodles do work.
An alternative:
Upon rebuilding the seats on the 280sl - I had read about using zip ties on the “S” springs and wanted to give that a try.
I used Stainless Steel zip ties. (Got them through Amazon, of course)
I tested the firmness (seat on garage floor) after adding a few zip ties to determine how many is enough and how many is too much. You can clearly make it too firm if you make them too tight or put too many. I used about 16 ties per seat bottom.
Between using a piece of carpet as suggested by some and using the stainless zip ties, the seats feel amazingly firm. Was pleasantly surprised at the difference.