When I went trekking in New Zealand last year the UA agent gave me a plastic bag to wrap the pack. It was large enough to fit the whole thing and tie off the end; we made a slit near the top to attach the tag to the center strap (i.e. not the belt but the small piece of webbing for hanging the pack) and to make it easy to carry. Did the same on the return with NZ, though I had to request ithe bag.

You could do the same with a clear plastic bag of your own, but to be of any use it should be thick enough to resist tearing, in other words not an ordinary garbage bag.