Ah ok so I found the best way to do this. Basically you open up the stairs, not the door like usual. That way no zombies will come down the stairs (these are the only ones that seem to hit you, all the others go for the guy 'upstairs'

. Then you just sit by the stairs, the box, whatever, and you should be fine.
Some people probably know that already but I couldnt find a tutorial on it and no one else told me, so there you go.