So after drooling on the Hard Graft bag for a while, I came across this article:
http://thetechedit.com/2012/11/why-i...a-macbook-pro/
All the same applies to a 13", so that's a big no no.
With that in mind, here is a potentially great alternative:
http://www.knomobags.com/usa/padstow...case-sand.html