Thanks for the suggestions and the info. After looking at all the options, Outsak looks like the winner. They are sold at Grand Canyon, it is less expensive than ursack, and it has a 1 year warranty. It also looks like I can rent one at the South Rim from the General Store.

Rent and try before you buy. I learned that with some gardening equipment years ago.

The guy I talked to at the general store said they have been renting them for some time now. They must work.