Growing up on a farm, our tractors had a cyclonic pre-filter. This was a plexiglass bowl with a cone inside it. As the air was drawn in at an angle, it forced to swirl around the bowl, dropping a substantial amount of the dust out of it. Such a thing would be pretty easy to improvise.

Several of my older small engines have a two-stage filter. The paper filter is the final stage, but this is covered with a thin foam sleeve that is lightly oiled. It's amazing how much the foam pre-filter catches -- and how long the paper filters last as a result.