DIY web site Instructables details how to turn heavy duty fabrics into a sealed, malleable bucket. All you really need is the fabric, then it's time to flex your cutting and sewing skills. When you're done, you should be able to fold the bucket for storage and just as easily return it to its full-on bucket state—kind of like the collapsible water dishes you've seen for dogs.
Collapsible Bucket [Instructables]