So, I need a minimal laptop-only bag (I've got a crumpler for the heavy days) to lug around campus this semester. I've got my eye on the Incase sling pack, which says that it fits everything from a 12"PB to the 17" MBP. Theres quite a size difference between the two, and since I've got a MB, I'm concerned that the sleeve might be a little loose and not hold my MB in place. Does anyone have one, or have they seen one? Is the sleeve inside bound with elastic? Or would a MB slide around inside?