Bike Friday has a seat cushion https://store.bikefriday.com/product_info.php?products_id=19127
And Amazon has these things, although I'm not sure if they're meant for adults https://www.amazon.com/Lixada-Bicycle-Leather-Cushion-Children/dp/B06XKFMPRR/
How about a rear seat attachment. Do you think this would work / be safe?
https://www.amazon.com/Lixada-Bicycle-Leather-Cushion-Children/dp/B06XKFMPRR