I have a whole container of sheets of plastic and cardboard packaging just for this purpose. Sometimes I use a hot knife to cut plastic, but this one was thin enough to just fancy cut with scissors.
The idea was to try to recreate the oak tree stencil I cut out of a piece of paper when I was gelli printing with my friend's kids up in Virginia. Elizabeth, quite the artist herself, created this amazing gelli print using it and I doodled on it. It is still one of my favorite pieces so far.