Got some pretty good ideas there. I like the gallon drink container...then I would put it in 2 or 3 plastic shopping bags with several layers of paper towels on the inside against the pitcher. Then I would put all that inside a handled carry bag. If you don't have a canvas or nylon one you might have to toss, a handled shopping bag might do it. The more layers the better. If the original container leaks, you have the paper towels. The shopping bags should keep it contained but in case it doesn't you have the handled bag. I would probably line it with kitchen towels too. That would the container from moving around too much.
My motto is 'if you prepare for the worst you rarely get it'. It usually is true.
One time I took cut up watermellon in a large bowel, tupperware. It leaked and I had put my canvas carry bag over my shoulder so I leaked the juice all down my back and the floor all the way in from the parking garage. I speak from experience.