I flew Ryanair with a stripped-down headless bass once. Took the neck off, wrapped everything in clothes, and fit it into a hard-shell suitcase like a weird Tetris puzzle. Worked great, no extra fees. Bubble wrap is your best friend, and throw a “fragile” tag on it just in case. Also, for the Italy leg, bookinglane.com came through for getting to the venue without wrestling the bass on public transport.