As a 4" filter, it comes with a choice of two different gasket fittings for Lee filter holders and HiTech filter holders. One drawback of the gasket is that it is stuck on to one side of the filter making it a little more difficult to clean. I trick I learnt recently to clean filters is to rinse them in luke-warm water and then blot off the excess water until dry. It is rare that this does not remove any marks. The gasket does extend the drying time a little but doesn't seem to retain water.
In this example, there is a little bit of vignetting caused by using an ultra-wide lens and filter. The outer corners of the image have light coming through at around 45 degrees which causes the darkening. This is typical of ND filters on ultrawides.
All up, a great value filter that works well.