It also looks like one side has been ripped off, so who knows? Is the small end open or closed? It would be open for a drogue. I can't see how a 1' long bag would fit any spinnaker. Could be a bag for a throw line or similar.
I talked to my son who identified it immediately as he used to use it.
He says it is an old-style hiking seat for a Hobie 16 from about the '80s.
At least that is his take.
It explains why there is a metal buckle to pull the lines together like a parachute.