Shells that don't have the outer protective film?

[QuackingPlums]

What do you guys do when painting shells that don't have the protective outer film on them?
I've tried cling film, which does a good job of keeping my fingers from smudging it, but isn't a very good surface on which to scribble my design with a Sharpie.
Also, do you tend to put the window masks on the outside and use them as a guide, or on the inside and spray/sponge the liquid mask over the top of them?