I've seen filters for other models, but not the recent ones. And they all ruined the look of the machine (They were basically just what you're doing...filter material taped over/under the openings.)
If you do use a filter of some kind, though be very careful: If the filter gets clogged with dust, the airflow will be restricted and the machine will overheat.This would be far worse than a little dust in the case. (In fact, I'd be a little worried about restricted airflow with something as thick as a Swiffer cloth. If I were you, I'd monitor the internal temperature carefully for a while to make sure it's not too high.)