I've been using a WaterField (http://www.sfbags.com) 1st generation RacerX bag (with mountain bike grip) to transport my 17" macBook Pro. The bag has a molded plastic case in addition to the inner padding for the computer compartment. I use a single bungee looped under the front lip of the rack, then reaching behind the bag & finally hooked into a couple of holes drilled into the side stiffening wings of the upper seat. In the picture below, you can just see the fluorescent yellow/green leg straps looped (with extra velcro) through the wheelie bag handle slot.
I've used this basic technique for about 4 years now commuting to work with no problems, originally on my homebuilt SWB recumbent & now for the past year on this Trek Y-11 conversion. There is no aerodynamic penalty since the bag width is a few inches less than my shoulders.
Keep the rubber side down,
~mrZ