Can your sleeping bag open up completely flat... or close enough?
Then lay down at one edge of the material... and roll yourself up like a Fantastic French crepe'.
At least that's how I woke up this morning. ::eyepatch::
Also, if you have some pliers or something you could try squeezing and adjusting the zipper slider (if it's the slider thats the problem...)
so it binds the uh...teeth? together better when you try to close it.
Or you can try scavenge a new slider from somewhere/anywhere else.
Good luck.