I use a Twitch too, and I picked up the Namba Lil Remix backpack, which I LOVE:
http://nambagear.com/lil-namba-remix-backpack/
It fits the Twitch with a Decksaver on it (killer protection, I never have to worry about anything being crushed) plus my 13″ Macbook with a shell case pretty easily in the large pocket. It gets a little tight when I add in my Crane laptop stand back there, but still closes and feels secure. The front pocket holds a plethora of cables plus headphones, pens, business cards, etc easily, and with a lot of specialized pockets. It feels safe and protective, and, bonus, it fits under airplane seats. I picked it up after reading Phil’s review here, and I can’t recommend it highly enough. Not too expensive on some websites right now either.