Epoxy, Fabric and Wood

Often, a swivel type clasp is used for securing cushions to the sofa proper.... in your case the plywood.

Probably the springs have not weakened. Rarely do I see broken springs. Most likely the spring supports have failed in some capacity, not the springs themselves. I see and fix this fairly often with older furniture. Spring support failure cause: 1) clips loosen or detach from the wood frame 2) Spring ties (string/twine) loosen or break 3) jute webbing or other banding straps loosen or its attachments (nails, staples) fail 4) a broken or loosened board (frame member, wood joint). None of these repairs are difficult, but more time consuming.... upholstery, in general, is really simple and easy work. For these internal fixes, it's just a matter of diving into the internals to access the problem..... just awkward maneuvering often times. We did a ply-sheet fix for Mom's sofa seating (spring support failure), until I fixed it later.

Sonny

I have done some upholstery repair in the past and as you say, it's not difficult. However, "it's just a matter of diving into the internals to access the problem" is a tad simplistic.

"Diving in" is easy. It's the "climbing back out" that poses the problem for me :-)

I dove right in to open up this box spring to get it up the stairs. Once it was upstairs, closing it back up was as simple as a bunch of staples, but the sloppy closure method will remain hidden until the mattress is moved again. IOW, if the broken part is not accessible from an area that will be hidden from view, I'm screwed. :-)

Mattress diving is not upholstery.

This is mattress diving.... in college we competed against one another in seeing how far we could mattress dive. We would wet the dorm hallway, place a mattress down, run and jump on the mattress to see how far we would travel.

Sonny