Whenever I fly anywhere, I have to take both a banjo and a fiddle. I usually get my banjo on as hand luggage, and my fiddle goes in the hold. I put it in a small case which then goes in my holdall inside all the clothing - it's worked fine for several trips to the States. Mind you it's only worth 3 or 4 hundred pounds. If it was a Strad I'd have to think of something else....
Ray