I got this from Amazon: http://www.amazon.com/gp/product/B00...ef=oss_product

Cheap M-Wave panniers. The top pannier is basically a large laptop bag. It fits my 15.4" with a lot of room to spare... just eyeballing it I think you could get a 17" in there with change. I'll try and remember to take it to work with me today and see if the 17" laptop at work will fit into it.

They're cheap panniers, but they've held up very well. Not rainproof, but it also has a rainfly (sold separately) you can buy (also at Amazon): http://www.amazon.com/M-Wave-122319-..._bxgy_sg_img_b

As an aside, I got caught in a surprise storm once without the rainfly... I don't know if I'd be comfortable with a laptop, but my clothes were only a titch damp at the bottom of the bag. If you live in a rainy area, I'd definitely recommend having the fly with you at all times. But the fly has been sufficient for me (I did touch it with Scotch Guard for added protection). With the fly, I wouldn't worry about sensitive electronics back there.

Being on top of the rack, it seems less worrying to me that the bike would crash in a way that it could get knocked about too much. I suppose the side could slap against the asphalt, but if you pack it right I think even that possibility could be averted.