I've been flying with my bass in a hard foam flight case for the past couple of years. The one I have is similar to what is pictured here:
http://www.kensmithbasses.com/doublebasses/Amati/
With my bass in it, it weighs in at 49 pounds so I'm never overweight.
I bring my plywood bass and make sure it is heavily insured etc. etc. These cases are super light and I'd say that I only get charged an oversize fee a little over half the time. The flip side is that they aren't nearly as sturdy as a gage or stevenson trunk. My personal feeling is that you're taking your chances no matter what case you put it in and so far I've had pretty good luck.