I'd be inclined to use a thermarest trail seat that can be partially inflated and rests against the inside of the pannier with a small section of folded over at the bottom for the laptop in it's sleeve to sit on then clothing and soft times packed nest to that. Or maybe a trail seat and sport seat duct taped together. I wouldn't bet that at some time your bike doesn't fall where you don't want it to. Either way putting the 'puter on the derailleur side my be some precautionary practice. I'm not operating from much experience other than a few miles and that's what I did.

http://cascadedesigns.com/therm-a-re...l-seat/product