In this era of solid-state storage, I'd be completely non-fussed about it being shaken around[1], so it's all about waterproofing, scratch-protection and not concentrating pressure on it in a crash.
On the odd occasion I've carried a laptop it's usually been in my Brompton C-bag, which has an appropriately shaped compartment, and a decent amount of physical protection afforded by the bag frame, or it's been in a regular pannier with some bash-protection afforded by tactical wrapping in civvy clothes. I do carry my 10" tablet + keyboard around while touring, and that just goes into the internal compartment of an Ortlieb front-roller with a dry-bag for additional damp protection, though I try to arrange it so that the back of the keyboard is in contact with the dome nuts attaching the lower pannier fixings rather than the back of the tablet.
A neoprene sleeve sounds like a good idea to me.
I would say that if your panniers are prone to coming off in anything other than crash conditions, it would be prudent to get better pannier fixings. I also hear that theft by grabbing the quick-release handle of Ortliebs is a thing in That London, so there's an argument for attaching it in a way that nobbles that (eg, by feeding the top strap round a bit of rack, or crossing the closure buckles over with a matching pannier on the other side of the rack, or even using the locking cable accessory).
[1] When I've had to carry hard drives by bike, I've used a rucksack.