What's a good bag for DSLR, small lenses and letter/A4 paperwork?

Looking for a bag that maybe is a bit like the Tamrac 3537 messenger bag:
http://www.amazon.com …ohs_product?ie=​UTF8&psc=1

but I need a flat compartment in the back that fits a US letter or A4 stack of papers without killing the corners of it. I guess that automatically means it would fit a macbook air, but I don't need any bigger than that.

Equipment wise I would like the same as that Tamrac, which fits a 200mm prime in the left, a full-size flash in the right and the camera with a short/medium zoom.

You might be better off with a backpack with a laptop insert. I have a few sling bags but there is no real way for me to put papers in them without folding or crumpling them.