Update: The Accurasee Sketch Caddie is back in stock, and now comes with 2 sizes
This is the Accurasee Art Pencil Case or Sketch Caddie. It's made to be used for vertical sketchbooks, so horizontal sketchbook users are out of luck until another model comes along.
There's a wide rubber band behind for strapping to your sketchbook cover. You can also use it on its own like a large flat pencil case.
There are three pockets and three slots for pens. The top pocket is the largest and can hold around 7 pens, including large waterbrushes.
The three slots in front of the case are for quick access to your pens. The rubber bands are tight and there's not going to be problems with pens becoming loose.
On the left is a small pocket with velcro that's good for small items like clips, USB drive, camera memory card or even a small collapsible brush. The right side is a see-through pocket with a zip. I put an eraser there so crumbs can fall through.
The pencil case feels durable with the thick canvas material, and yet isn't too heavy, although if you pack in a lot of stuff, you will weigh down the cover of your sketchbook.
Update 11 Feb 2014: There are now two sizes, small for wrapping around 8.5 to 10.5 inches, and large for 10-12 inches.
It's available at www.amazon.com/gp/product/B005QSTJYO.
This video review covers what I've said above.