I think Cindy's solution is the simplest and most practical. Even if all you do is find an old frame to stand in for the trip.
I was once in a class with a student from out of state who had located some little devices that looked like push-pins with pins on both ends. He put the pins in the corners (which the frame rabbet would eventually cover) of both right sides of the wet paintings so that they faced each other, then wrapped them with brown paper and tape.
You could do the same with some stiff cardboard. I don't recall ever seeing the pins, but then I haven't really looked
.