Re-Foam Attached Cushions Settee?

[fender62]

my has a leather seater settee, cushion on end is sagging in, i assume it needs a new piece of foam, problem is the cushions are not detachable and have no zips, so i upturned it and took off the bottom liner to to have a look for him, i cannot for the life of figure out how to get a new piece of foam inside, how thye built it ermm i have no idea, probably some mass produced thing of staples and glue, but i do like to tinker..any ideas..

[Buenchico]

Any use?
https://homeguides.sfgate.com/redo-missionstyle-couch-66255.html