Dingo, get a small piece of square metal and grind it down to the shape and size you need, then take a dremal or die grinder and hallow out the inside of the square to make it sharp. set the vent on a piece of wood and smack it._Ron
I can send you mine if you give me an address......you can design them up and I will be your guinny pig!
Not so sure a "Diamond shaped Punch" will work though....this plastic is really hard! It would take a lot even to do it my way on a Grinder.....I tried shaving some off with a course file....That did not work!