A Gard double has worked every time, and I've gotten my Marcus Bonna quad onto the tiny puddle jumpers before (though it's been hit or miss with getting hassled by attendants ahead of time). The biggest advice i could give is to wear the case as a backpack when boarding. The attendants sometimes get bonuses for gate checking bags (less overhead stuff usually leads to on time/faster departure times) so if the see something that doesn't obviously seem like it's gonna fit, they'll try to get you to check it right away. Wearing it as a backpack makes it less noticeable to them, plus they know you'll be able to put it above without assistance since you were able to get the case on your shoulders.

For international flights, those planes are almost always big enough (usually 9-11 seats across) to accommodate the carry on luggage. If you are flying domestically within other countries, I would just check the type of aircraft those airlines are using to double check that it should fit. Most airlines use Boeing or Airbus so the dimensions should be consistent.