I went out and bought a 2" thick piece of foam long enough to reinforce the crushed down left bolster. This time, I completely removed the seat from the car, so that I could sit on it after I had done the reinforcing, but before putting it back in the car. Then I put it back in the car. It feels as good as new!
Of course, as these things go, when I turned on the air conditioning today before coming home from work, there was an awful vibration that appears to be the blower motor. (Not dependent on A/C being on; directly proportional to A/C fan speed.) A review of Corvette forums suggests that it is either blower motor bearings failed, or (in light of past history), a mouse nest. If so, the little critters would have had to have hauled something heavy or stiff into the blower.
I went deep enough into the process to decide that this is something worth paying a professional to do. I can't see the bolts that hold the blower motor in place, and I am not at all keen on the weird gymnastic position required to look under the glove compartment to figure this out.
UPDATE: It did indeed turn out to be a mouse doing what might be called a zombie version of a hamster wheel in the blower. Removing the remains and desmelling it all will be about $150. I think I will stop leaving the top off the car in the garage. Our cat just isn't good enough at doing his job.
UPDATE 2: They forgot about the deodorizer for the A/C system (which since it adds $80, is probably something that kills bacteria, too). The mouse was apparently lodged between the fan blade and the housing. No, he was not in a position to comment about the experience.