Steel pipe = "pipe bomb" was my first thought too but I let it go. Given JohnInKansas' details of weights I thought I'd measure up a "PVC pipe" model that already has airline stickers on it and no damage.

It's 850mm (33.5", to you imperial measure users) long, 90mm (3.5") wide and, with a foam plug in it left over from whatever was last transported in it, weighs just under 800grams (1.76 lb); one of its ends is permanently sealed and the other end has a threaded cap. I think this is large enough for most bows but, if I were trying to protect a fiddle bow I'd probably wrap the bow in polyethylene packing (like museums and some computer screen manufacturers use) to make a snug fit inside a length of 38mm (1.5") PVC conduit and put that inside the 90mm PVC pipe with the usual foam packing between the two pipes. Gaffer tape would prevent the threaded cap from undoing but allow inspection.

I don't know the USPS rates (too lazy to check out JohnInKansas' clicky) but I reckon it'd cost less than anything with metal pipes and not arouse as much suspicion. A lot more transparent to X-rays, too.

Cheers, Rowan.