A larger size of the Tiny Shingles Stencil (same small holes... just a larger overall stencil so you can use on larger items, like the widest tail of the Articulated Mermaid Paper Doll shown in second photo).
If you're planning to use the stencil a lot, I would use spray shellac to seal the stencil before you get paint on it (an oil-based varnish, rather than a water-based acrylic spray will last longer). Spray a few layers and let dry. Flip it over and do again.
Teri Calia made a Dia de los Muertos Cat Mermaid Shrine (see the wide photo below). More info in the blog and/or see the supply list.