I assume that sling means a shoulder bag?
For the Motion, I use a Willow Designs Iwhumph. Its a sleeve/shoulder bag. Hard plastic on all sides/top/bottom, thick padding inside. Perfect fit. Holds the unit perfectly. Keyboard will fit in snuggly. Front pouch for my VIK, mouse, battery and adapter. Thin pocket in back for slim folder and paper.
This fits into my Timbuk2 Detour bag. This is either a shoulder bag or backpack. Padded laptop compartment fits the Iwhumph perfectly. Gives me more space for books and binders. Back is padded making it pretty comfortable.
Then if I really have to lug some stuff around, there is my Timbuk2 El Bolo. Gigantic. Fits the Detour/Iwhumph, and adds room for another 6-8 books/binders and my lunch and dinner. This in total also provides me with backpain later that night.
Iwhumph is shoulderbag/sleeve.
Detour is shoulderbag/backpack (with weak laptop padded compartment).
El Bolo is shoulder bag.