The quest for comfort continues.
I was about to return the Cooling gel cushion (model HA9255, also $20) i.e. the one that actually doesn't contain any gel. It is described as a "Cooling Gel Seat Cushion". Although there's no gel, what it DOES have is a rubbery honeycomb sort of pad about 20mm thick glued to a fabric backing which breathes. The rubber feels like a tough silicone rubber.
I wondered what it would perform like with the gel bag from the MA2488 under the rubbery pad so I slit the foam, extracted the bag, unzipped the HA9255 slipped in the bag and went for a drive. As it is thinner I did not have the feeling of being perched up on the seat as with the other cushion and there was no need to trim foam. Being black it looks better too.
It did a similar job of damping the jarring/jiggling. It does not feel bouncy but it was not as "dead" as the MA2488. The bag is rectangular and the gel was soon pushed forward and bulged up near my knees. I had the bag fitted fore-aft so I pulled it out and fitted it out smooth (no creases) across and towards the back and that worked fine. I deliberately drove off the side of the road where there was choppy disintegrating bitumen which would have had me gritting my teeth and the improvement was quite significant. So far I would not be without it but long-term and for long trips we shall see.