On surface a steel pipe sounds like a good idea, however my fiddle bow will not fit into a 1" diameter hole, so at least a 1-1/4 ID pipe would be needed. That's not commonly available in my area, so it would probably be a special order. Minimum wall thickness for common pipe would be very close to 1/8." My bow would need a 30" long pipe to allow any end padding.

That's about 16 cubic inches of steel, or about 5 pounds. Screw on end caps will add another half pound for two caps in common style.

If you took the "easy out" and used the more common 1.5" pipe, the available wall thickness for what's easy to get increases slightly; but calculating at the same 1/8" thick, your pipe is now 9.5 pounds (with end caps) all by itself.

Using the USPS Rate Calculator:

From my middle of the US location in Kansas, to New York or to Los Angeles, the lighter 5.5 pound pipe would mean paying about $32 postage for "Express" delivery which is the closest one can get to "First Class Mail" for an item of this size. Parcel Post, which estimates 6 days in transit is about the cheapest thing available from USPS, at just over $10 just for the pipe. The larger pipe would be about $40 Express, and just over $13 for the slow parcel post.

There might be an additional problem with something looking like a steel pipe when they xray to check for "suspicious packages" - especially if "hidden" inside a suitcase as has been suggested.

(Postal inspectors have virtually no sense of humor.)

John