There is a small gap between the filter and the edge of the opening. Filter will fall down when system turns off. Then get lifted back into place when it turns on. But not perfectly flat so it will vibrate a bit during the time that system is running. Need to find a filler for the space o just get the original filters again.
I love our custom filters. Our home has a very unusual size for the venting which requires us to obtain custom ordered filters. They fit "perfectly" and we can now change them out every three months with a standing order!