Ken Hulme:
When I ship dulcimers, I use the ultra heavy (1/2" thick) cardboard tubes used as concrete molds for piers and posts, which you can find at Home Depot or Lowe's. It costs a couple bucks, but that's not a problem. I cut the tube a few inches over length and make end caps from corregated cardboard and tape one in place with balled up newspaper or whatever for end padding. Inside, I roll the dulcimer up in bubble wrap until it's a snug fit, side it into place, then tape on the other end cap. Because of the shape, any weight place on it or shoved into it tends to be deflected. Either that or the carrier stands it on end.
KEN: Thanks for this idea. I see these tubes come in different widths. I'll take my dulcimer to Lowe's to check it out, but what do you usually use?