I have seen those for sale at rallies. Don't know the name, but they are out there. A company also sells pads made from the material used to prevent bedsores on people confined to wheelchairs. I think they are called Airhawk. Both are non-skid on the bottom and use a strap under the seat to secure.
Gel seats work great for long rides. The trick is to ride as long as you can without it, then put it on for the duration of the ride. As any seat becomes bothersome after hours and hours, so will the gel seat. IF you mix it up like I said, your hiney will greatly appreciate the variation ;D