there's a great guitar case called the fort tric case, made for seagull, guitars. it's made of the same stuff that's inside car bumpers, some kid of very hard dense foam. it's light, waterproof and tough. it insulates extremely well.
http://www.fortstands.com/forteppcase.htm
It seem like something like this might be good or a double bass. On the other hand, I actually don't have room for what would in effect be a second, larger bass, and it'd probably be harder to schlep in and out of gigs