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By "flock", I assume you mean apply some kind of opaque covering to the inside/outside surface. I have tried painting translucent PLA cameras, and it's an ugly hack. There's a guy on Thingiverse who has printed a bunch of my cameras and he wraps em in gaffer tape.
This kind of silliness is unnecessary: OPAQUE PLA EXISTS. I am currently using eSun Black PLA and Matterhackers Jet Grey PLA.
"Flocking" not required. The eSun prints as well as any PLA I have ever used and is less than US$25 / kilogram. You should be able to leave a camera, loaded with film, sitting in bright light indefinitely without worrying about your film fogging and destroying photographs.
For the record, I do apply a flat black acrylic on the inside surfaces of the camera that could potentially cause internal reflections. But I don't rely on paint for opacity.