One approach to shipping larger works when you can’t find cardboard tubes or boxes to fit your painting, is to buy PVC tubing in the plumbing department of your hardware store and have it cut to fit your painting. This approach is a bit more involved. You want the tube to be a bit longer than your painting. You’ll need to buy caps for each end of the tube. In order to attach the caps on each end you’ll need to glue them on. [Photos of all supplies used are at below these videos].
Example of large canvas painting (60″ x 72″) shipped across the country