John you asked about a dust mask for your dog. Well the first idea that hit me was start with a muzzle. After all they are made to fit around a dogs mouth and nose. Might not be a happy puppy but. Then take filter paper and cover the muzzle. You might have to piece something together from people dust masks or coffee filters. With the coffee filters you might need a double layer. Then have several so it can be replaced. Not real sure of the best method of hooking filter to muzzle that would take some trial and error.

Not sure if it would work but might be a place to start.