Been flying many times over the past years with my big MTB. Best is to use a thick cardboard box that you buy for a few bucks at your LBS, the one in which bikes come from the factory. Wrap up any fragile parts (forks stanchions, carbon bars) with basic insulation foam tube. Finally, cover the box with plenty of "fragile" stickers.
As opposed to a dedicated bike bag, the slightest impact on the box would be visible and the airline will take great care of it.