I had vignetting the first time I used it. I took the filter off, reinstalled it - It slid on much easier after having been on the machine a few days. Turns out the problem was that it wasn't pushed all the way on. No problems since. Try moving it back until it bottoms out on the P3 Lens.
Just to be clear, it was bottomed out. The still photo in my video is post-cutting. After cutting, the filter bottoms out with the ND glass touching (just about) the front of the camera. Previously, the black rubber would hit the camera body, keep the ND glass about 1/4" from the front of the camera lens.
I just completed my first flight with the trimmed filters. The live feed seems to show no vignetting.