I was very happy with my first cover and that is why I went back to my dealer. I really didn't want the hassle of measuring and then worrying if I measured perfectly. I also was concerned about how to get my old cover off the lifter and the new one on without messing up the lifter. The technician at my dealer's walked me through it. I know that if there are any warranty issues, I will also have dealer support. So maybe I am paying more for all of that, but it was worth it to me.
Lots of people on here have had great success ordering on line. Maybe Doc will chime in and tell you if they have specific templates that will guarantee you a perfect fit. I am sure you can save money that way...
Just curious..how old is your current cover and do you know who makes it?