How exact does it need to be?
I use a Sawzall w/a course (Shrub pruning blade) to trim them. I have 2 ATVs that don't like full sized paddles...a 200 and a 230. Cutting them in half really helps w/them being able to pull the paddle in 3rd, 4th, and etc. Of course, in 1st gear, I can't climb...