I just bought an one shoulder backpack at
walmart for $9 canadian. It's big enough to hold our Passporter, cameras, camcorders, extra batteries, ponchos, sunscreen, drinks and snacks. A change of clothes is an excellent idea and I'm sure with using the ziploc trick you could fit it in. We got two school age kids so we might have to go back and get another bag..
It's designed to go over the shoulder like a purse. Let me do a quick search and see if I can find you a picture.
Here's a link to one similar to the one I bought
http://www.walmart.com/catalog/product.do?product_id=3208707