I got a laptop bag with a padded inner case for the laptop, heavily padded strap. For clearly too wet, I'd put the laptop & the inner case inside a plastic bag, although I never actually had water pentrate. Sort of a super messenger bag. I don't recall the brand right now - the thing is at home housing my dead laptop. This was stable enough.

I'm good for about 15 miles with a messenger bag, then it starts to bother me a bit. I'll stop and change shoulders. Does impact my position somewhat. I've got my commuting bike set up with a bit higher bars. Balances me better with the bag. Still working on commuter position.