From one designer to another...
Hey there Jon. I've been traveling with my 13' macbook and it's been holding up great for the last 2 1/2 months of steady travel with the adobe suite.
I of course have a small soft bag that I keep the laptop in, which by the way is a perfect fit in the side hard cases hatch. Other than that, I toss the comp bag in a military dry bag which you can pick up for 5-10 bucks at any surplus store. The dry bag take up no more room than a garbage bag.
The comp casebag seems to be sufficient as padding and the dry bag keeps it dry. I've been through some serious rain and so far so good. It's cheap and it works. Of course don't forget to turn the comp off before riding.
That reminds me, I should do some back ups...