kf5nd, you're my kinda person.

I made a would-be laptop messenger bag out of the same stuff, plus foamboard I had leftover from a display project and heroic amounts of duct tape. I braided three tire tubes together and settled the resultant box inside them, duck-taped it to the rubber, and the resulting strap is the most comfortable I've ever used. Total monetary expense: $0. All from used materials I had on hand that would have been thrown out anyway.

My other stunt is I did indeed get one of those laptop carrier backpacks, except mine was the rolly kind. It's ok on the back, not excessively comfortable though. Talk about a posture enhancer! That aluminum frame kept my back like a ramrod.

Now I attach the rolly handle to my cargo rack with a pair of heavy-duty carabiners and trailer it behind the bike when I need to use it. It creates an unholy racket, but I've a plan to remove the small rolling wheels, bore out the brackets that hold their axles, and put in a larger axle attached to larger squooshier wheels. This plan is awaiting the arrival of said larger squooshier wheels, but aside from the racket I"ve never had a problem with the setup.

I do prefer to make the bike carry the load, though. I find it much less wearying on me, and there is a certain mechanical advantage given to everything lifted and pushed through the chain and cogs. I'd prefer to have that mechanical advantage aplpied to the cargo too, as it doesn't to my back. There is indeed an overheating problem in summertime, when one's back isn't avilable to vent excess body heat. It is indeed more comfortable to ride unfettered.

Oh, BTW...by bolting upsidedown U-shaped brackets to the back of any stiff-backed laptop case, and an adjustable hook on the bottom for security, a normal pack can be made into a panier. Measure the case and rack to be sure where to attach everything, and you shouldn't have a problem.