Thanks for all the suggestions. I have an ortlieb office bag actually. Their laptop sleeve (which I don't have) seems like it might be a bit baggy on the 13.3" screen computer I have coming (vs not fitting at all the 17" beast that is being replaced). If it attaches securely into the office bag however that might be a tradeoff I need to make. I have another waterproof pannier that I throw the computer into (vertically oriented) if I'm not bringing the office bag.

My current laptop is huge and I don't carry it much for that reason (bulky and heavy). The new one is going to be smaller, more portable, and more expensive, so carried more and yet more vulnerable. Maybe I should just stick w/ ortlieb's sleeve and anchor it into the ortlieb bag.