In the past I made them from freezer paper, but this requires more effort from the guests to iron them on to the onesies. Then I made them from cardstock which works fine, but you can only re-use them for so long. This last time I made them out of plastic and they work like a charm.
I just added a listing to my Etsy shop with a set of 8 to make onesies for baby girls. I should have two more listings up within the week for a boy set and gender neutral set.
Below is a picture of two of the stencils cut out of cardstock so you can see them.