Bikeroo (like Kangaroo)!
4.5 out of 5 from 650+ reviews as “Improves Comfort for Mountain Bike, Hybrid and Stationary Exercise Bike”
Most reviews I’ve read and listened to say peddles, grips and seat are the best and cheapest upgrade. The saddle having the greatest impact on comfort.
(I hear carbon rails have give, but an oval shape, so different mounting).
Found this one after extensive search. 31 bucks on amazon dot com.