I probably didn't invent the wheel here, but the other day I needed to bring some important files home from work and didn't have my usual waterproof document bag with me. Went to look for a plastic bag, but could only find the flimsy liners that go in our office trash cans. Walking back to my office I passed the mail room and noticed a stack of large parcel envelopes for fedex and UPS. The UPS envelopes are made of waterproof plastic, and the fedex packets are made of waterproof tyvek! Light, sturdy, waterproof, free. Fits the bill.