I have a home made pack cover that opens flat. Essentially a rectangle of nylon with a draw cord around the edges. I've used it for a pack cover a handful of times, but more often used it as a drop sheet, a gear hammock, a firewood collecting tool, a couple times to get a bunch of water to put out a fire, and to throw over my head running out to grab something that blew away from me while tarping. It's also kept the pack from being drenched walking through a falls and once in the rain.

I am migrating away from using it and more often use a pack liner, and the poncho does the rest.