I have a Wald 585 basket attached to my rear rack via stainless hose clamps. It would accommodate a briefcase just fine, if held down from above by a bungee. When I transport delicate things, I just wrap them in a plastic bag to protect them from the elements, and I leave the bag with the bike when I park it.

I think it's worth looking for creative ways to deal with having to haul a giant laptop computer back and forth every day, if that's what you're doing. Naturally this depends on the nature of your work, and what's on your computer.