Is there such a thing as a vent filter?

I do not know how helpful this is, but have you thought about an air filter for a car or truck. They come in various sizes and you would need to get something close to the size you need then put it in place (on top or underneath the vent) with your engineering-spec multi-purpose adhesive. I do not know how restrictive these filters are either but it should be relatively cheap to try. Good luck and let us know what you do and how it works.