One that I have seen mentioned many times on Bentrideronline is the Ventisit pad. They are model specific and not inexpensive. You might look there as there are some posts about rider's experience like this one from 2016 https://www.bentrideronline.com/mess...d.php?t=128176 Although mostly made for ventilation I guess they also provide some comfort as well.

I used the foam insert on an Haluzak Horizon SWB to increase the comfort of the mesh seat on it, Cheap and effective.