There are several options for using polarizers or filters.
1. Keep the cover "open" in front with your filter instead of the Outex front port for water-resistant/above water use (rain, mist, sand, mud, dirt, etc.).
2. There are flat front ports (below 77mm) that offer external threads (77mm) for filters or polarizers on the outside for a waterproof solution (even underwater).
3. Or you can use them inside the housing if you're looking for waterproofed protection, but with limited access if it's a variable filter.