I use Arkel's Briefcase. This is the best laptop bag you are going to find. It suspends the laplop in a sleeve on the inside and the outer bag has reinforced plastic inserts for protection. It has plenty of room to stick other stuff in, and an elegantic system that covers the mounting hardware for off bike use. With two layers of protection you are pretty safe from the rain, but with the additional waterproof cover it is bulletproof. I used this combo all winter long in the rainy PNW.