I think I arrive at my destination with a sweaty back even if I only have one layer (which is what it will be on the way home.) I have seen backpacks that are raised off your back and use a mesh material. That would probably work fine for a one layer commute. I'm hoping my carradice works out.

SQR Slim saddlebag



My bag is not nearly that big, but that's a picture of the mount.