Flying Mono M80 Single...without the bass

It's gotta fit in the overhead bin if it's carry-on. You may get a kind flight attendant that puts it in the pilot's coat closet thing, but I wouldn't count on it.

 

I've flown with my M80 as a carry on, and the flight attendants never checked to see what was in it. Security might wonder why you don't have a bass in it, but they won't keep you from taking it.

 

Gate check it right before you board. The gate agent will likely want to do that anyway, considering the size. And there shouldn't be any fee. Yes, it will be stored under the plane with luggage (and handled by that crew), but since there's no instrument inside, there shouldn't be much to worry about. Those cases are built well.

 

On small planes it won't fit, on larger ones it will. On an international flight I would expect a large enough plane for it to fit overhead. However, it wouldn't be a big deal if it was gate-checked.