Have you considered a PVC hoop house that you can put up and take down with minimal fuss? You could put it up over your garden in the spring and/or fall to extend you season by a few weeks at each end.
These are the plans I used for the one I made.
http://westsidegardener.com/howto/hoophouse.html
How to seal up the ends was the tricky bit, but I overwintered several pepper plants, a couple of egg plants, and a mess of weeds one winter using this.