I have at times used styrofoam paper plates, and even better the plastic paper plates for targets. They cut very clean, and large holes. Stapled a black plastic gargage bag up once (to cover the many bullet holes in the cardboard backing). To my surprise, I found if you used the plates on top of the plastic bag you got the nice clean full bullet diameter holes, but the plastic mostly stretched around leaving a smaller hole. The resulting large clean hole at high constrast to the black plastic made seeing your holes at least three times easier. Sounds like this styrofoam board (available cheap at art supply stores) and covered with some stretchy tape probably resulted in about the same thing.
