The plastic I used is Avery 11900 Insertable Dividers (this also works great in a Craft-Robo!) To make your own: cut a rectangle about 1.5 by 3 inches. Fold in thirds the long way to make a little burrito, and crease the folds. Cut one end to a blunt-ended triangle as in the photo. Unroll the burrito, cut off the outermost triangle, and re-fold. Squish it between your fingers a bit so that it bulges a little, put the pointy end inside your lips and blow. With some practice you can get a horrible squawk out of it.