Try a trash bag. Sometimes something as simple as pulling a trash bag up on to your sleeping bag from the bottom will give you an extra margin of warmth if you're cold. I wouldn't do it on a regular basis because the bag won't breathe. I would use it only as a measure when you find the sleeping bag isn't quite cutting it and you need to warm up a few degrees more.

This is especially useful if you're group camping and someone discovers their sleeping gear isn't keeping them warm enough. A simple trash bag can often boost the warmth of their bag enough to help them make it through night.

Regards,

Tom