the shield will most likely be flexible. if so, you can just press it down flat on the table. i've used heavy duty scissors and xactos to cut welding shields. the "score and snap" method works well... make a cut with a ruler then bend the plastic and it will break along the line.
you should also have enough excess plastic that you can practice cutting around the edges of it. you can tape your template to the shield so that you know you're safe then hack away at the rest of it with whatever tools you like