Mannyrock,
I am not a modeler; however, I have had to make more than my share of holes in different materials. For this effort I would use a hole saw in reverse. By reversing the drill, you will be less likely to catch a tooth, but it will take a bit longer. If possible, try and back the plastic with wood, such as a piece of wood in a vise and then the plastic to be cut on top of the wood allowing you to hold the plastic while drilling down in reverse.
