Up until 2006, from 2002 I was flying to Spain and back regularly with an NS CRM5 on Easy Jet, they would let me take it on, if I booked a seat.
Initially I said "it's an electric double bass" they replied "no double basses" so, when I booked the seat, I said it was a cello, no problem!
"Mr Cello" as he became known for booking purposes, had to have a window seat though, apparently he wouldn't be much help in an emergency...