Skip the holes. You don't need them. I've cut my own sheets for YEARS
and only drilled holes once. Just give the clamp an extra push or two
as the plastic softens and the teeth will dig in just fine. Unless you
are molding something really tall or don't let the pastic get soft
enough, it won't pull out of the clamp.
You can also improve your results by modifying the molding platform.
Drill more holes in the platform. Not bigger, just more.
Build a gasket out of silicone or clay that fills the gap between the
edge of the platform and the raised metal lip of the vacuum "well".
The gasket should be about 3/16 wide and about 1/16 high. If you are
lucky, you may be able to find a weather stripping product with an
adhesive back at the hardware store that you can rip down to size. You
can also carefilly sculpt a gasket with tube silicone. The easy, but
messy, method is simply to lay a 1/8 diameter clay worm around the
edge of the platform. The gasket will both seal to the plastic and
plug that big leak at the edge of the platform. You'll be amazed at
how good a vaccum that hand pump will pull.
Greg Reynolds, IPMS