It was pretty quick to do. Took a photo, printed it out on a full page, glued it to a bit of cardboard box. Cut it out. Used an exacto knife to cut out the iris of the eyes, so the kids could look through the masks. Gaffer taped a popsicle stick to the back and then to make them a bit more durable I put some strips of packing tape over them.
Here's a pic of my two year old pretending to be my three year old with her mask.