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That's easy enough, cut some 1x2 and some 1/4 inch plywood to fit in a priority mail box. It would be pretty crush proof at that point. Although mail trucks are heavy.....

Foam insulation inside of hard PVC in a box would give you a very very high degree of protection. I'm sure there are many other ways.

Foam pipe insulation is great. 1/2 inch for up to 5/8 and 3/4 for larger works great. In a 9x12 manilla envelope rolled up. Never had a problem.