DEMCiflex makes great filters I have a full compliment of fillers on my M1 that has been running for almost a year and I have yet to clean any dust from the inside. They cut down 90-95% of dust if you have positive pressure in the case and clean them every couple of weeks or so. I have also used them in my previous build based on Define R5 and I only cleaned the inside once in 3 years I had it.
A little note though, they tend to size their filters on the OD of the opening mostly, I prefer to go with ID so not to impede airflow. You can do that by searching stock sizes and matching them to the dimensions of openings on your case or custom ordering.
I you are interested I can post pictures of the inside of my case, filters and exact specifications of what I ordered?