I have one that is similar in style to the P50. I also consider it plenty "bomb proof" enough for mailing. And I think they are used for that all the time. If I was concerned about weatherproofing, a high quality trash can liner would ease my concern.

If rolling is an option, you can build a tube from plastic plumbing pipe (use the version that is lighter - I forget what it's called).

You could always visit your local art gallery and see what they use.