This is a very niche case. I bought “PUREBURG Core Mini-RF Replacement Filter Compatible with LEVOIT Core Mini Air Purifier, 2-Pack H13 HEPA 3-Stage Filtration Activated Carbon” to connect to a 4" diameter exhaust. This snugly endcaps one end and allows connection to the pipe the other end. I expect zero other people to have use for this but you never know. There is a picture of just the filter without any endcaps on just to show what the filter looks like.