Wow! Great responses, folks. I actually thought of the "gig bag" before as an inconspicuous carry mode in and out of my apartment. Don't want to make myself an easy target to get robbed when I'm not home. Thought about getting a guitar case for an acoustic guitar and padding it with craft foam. But that's really only practicaly for one gun. I like the "big duffel bag/hockey bag" idea for range trips.
The hard golf case looks like just the ticket. Get some locks for it, and it would be a great way to check guns for an airline trip too!
Blackhawk makes some big rolling duffels specifically for guns, but I think they are a little too pricey, and still somewhat conspicuous.
As far as being worried about being robbed during the trip, I'm not (much). The guns would either be locked up in the car (out of sight) while I'm on the road, or in the hotel with me after I check in. I just didn't want to alarm any sheeple while the boomsticks are in transit between room and vehicle. Unless someone wants to try to rob my room or vehicle while I'm in it (which, combined with my heavily armed state and a reasonable amount of awareness, would be poor choice), my guns won't be there. Of course I won't be taking them all with me on potty breaks, but you can't eliminate all risk and still carry on a normal life. I'll just have to trust the lock on the trunk to work as my security for pit stops. If someone steals my vehicle, that's just bad luck or bad choice of rest stops.