Most people approach packing their bike from the completely wrong direction. Protecting a bike has way more to do with what's going in inside the box than with the strength of the box itself.
Try this thought experiment. Take an egg, wrap it in several layers of bubble wrap, put that in a standard cardboard shoebox and drop it from shoulder height. Your egg is still intact. Now take that same egg, remove the bubble wrap, put in a shoebox made of heavy-gauge steel and drop that thing from shoulder height. After you dig it out of the crater in the floor you'll find that your indestructible box has a broken egg in it.
What about doing both, using the bubble wrap and the steel box? Wouldn't that be even better? Sure, but the fact is even in the steel box 95% of your protection is in the bubble wrap, the steel is largely just dead weight. Now factor in that with a bike box you're never dealing with a container as strong as heavy-gauge steel or cargo as fragile as an egg.
Spacers in the dropouts, pipe insulation and/or bubble wrap over everything, zip tie it all together so nothing can rattle around (and the TSA only has one monolithic block to look at) and you'll be fine whether you've got a soft bag, a cardboard box or a fancy-pants hardshell case. Skip any of these and you're setting yourself up for a bad time regardless of what's on the outside.
Try this thought experiment. Take an egg, wrap it in several layers of bubble wrap, put that in a standard cardboard shoebox and drop it from shoulder height. Your egg is still intact. Now take that same egg, remove the bubble wrap, put in a shoebox made of heavy-gauge steel and drop that thing from shoulder height. After you dig it out of the crater in the floor you'll find that your indestructible box has a broken egg in it.
What about doing both, using the bubble wrap and the steel box? Wouldn't that be even better? Sure, but the fact is even in the steel box 95% of your protection is in the bubble wrap, the steel is largely just dead weight. Now factor in that with a bike box you're never dealing with a container as strong as heavy-gauge steel or cargo as fragile as an egg.
Spacers in the dropouts, pipe insulation and/or bubble wrap over everything, zip tie it all together so nothing can rattle around (and the TSA only has one monolithic block to look at) and you'll be fine whether you've got a soft bag, a cardboard box or a fancy-pants hardshell case. Skip any of these and you're setting yourself up for a bad time regardless of what's on the outside.