I was pondering the same thing a while back. I am using my regular back pack now, but if you can get one with a waist strap, that would be waaayyy better. I have to be careful the weight doesn't slide off to one side or down toawrds my head. I was doing them weighted with my feet on a chair but it kept sliding to my head so I had to go just flat. A waist strap would eliminate this problem, I would imagine.
I don't know how you would comfortably go heavy on these. I am goign for large sets of very fast pushups and will not need to add huge weights for a while yet.