Try finding a 126 in a junk yard with a firmer seat. The seats were not really designed for heavy folks. However the cars are great for tall. You could probably take the cushion to someone who does upholstery and have it redone! That would be expensive. I have not seen a do it yourself rebuild of the seats

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My Daily : 96 E-300 Diesel with 195,000 miles
Retired: 92 300D 2.5 T 345K miles and for sale
Retired: 95 E320 157K miles and currently parked with blown engine

Both retired cars are for sale as is my w124 shop inventory