In the age of plastic, one item reigns supreme. The Ziploc® bag!
It will hold just about anything you wish, including water! The only drawback to using or reusing, is that some objects may puncture the walls or with heavy items such as water, blow out the seams. To protect against this happening and to extend the lifespan, try the following:
Using the clear plastic packaging that much of todays electronics and other gear comes in.
Cut it to size with scissors and sand the sharp edges off. Alternatively, you could also tape the edges.
Use this for items that would wear thru in short order.
Use the lighter weight cardboard (think beer case) for other items.
Protect the bag from punctures by doing the following:
Hard plastic inserts (or thin card board, such as the beer case)
Several layers of butcher's paper would work also.
Duct tape the edges and bottom to prevent blowout.
To keep external abrasion to a minimum, cover the bag with a layer of duct tape.
This will save you a trip to REI to spend hard earned money on those fancy stuff bags when you could buy beer and reuse the cardboard instead!
Bottom's up!