I just watched a fons and porter show that adressed this. They put a small cutting mat on a large one...cut 2 sides and then turned it to cut 2 more. The other thing they did was use a glue stick to hold a template in place - the thin kind you can make yourself. I also have heard of gluing fine grade sandpaper to the template - that helps hold it in place and not slip - have not tried any of this, but it sounds good.