I have tried to cut it apart with a rotary zip saw but it is
taking forever

Never send a boy to do a man's job.Rent or buy a SawzAll and you can have the
thing sliced, diced and ready for the trash in under an hour. Most any SawzAll
blade will go through fiberglass readily. You could do it with a sturdy saber
(jig) saw, too.
Whatever, make the pieces small enough so the your garbage professional can
lift the container. They would appreciate it if you did the disposal over two
or three weeks.
HTH

Joe