I was wondering when this would come up but with a different problem.
I was looking at these one piece head
liners in the new boats at the boat show
a couple years ago and commented to my wife "How in the hell is one suppose to do any maintenance
fittings!" And basically said "I'd never own one until the liner had been pulled out".
What happens if one of the fasteners starts to leak or even fails? Especially at sea. What a pain in the back side trying to get one of those things out of there and then try get it back in.
I hadn't thought of the sagging problem. But it just confirms my dislike for the production pieces.
As for it's (temporary) repair; I like the spray in foam idea but some day it's all going to have to come out of there.