Many of the wide angled lenses don't fit standard filters due to the bulbous nature of their front element. I have the Pentax F2.8 16-35mm which has a bulging front that makes attaching the standard filters impossible. However, Benro make an adapter that works just fine. It does not screw into the lens but fits over the outside of the lens and then is tightened via twisting the blue band (see pic below). Into this you can slide the 150x150 square filters - I also use Benro for this to make sure they fit correctly. ( I use the ND 64 and ND 256 filters with this setup) This filter is marketed as being for the Tamron SP 15-30mm 2.8 DI VC USD, which is I think the same lens as the Pentax, apart from the coatings. What is not apparent though is the need to block out all light via a black sleeve that fits over the lens. This is necessary as is the cover to slide over the eye piece. Once attached this provides a light proof setup that makes long exposures possible.
Below are some shots of my Pentax K1 with the Benro filter holder attached to the Pentax f2.8 15-30mm lens.
This first shot shows the filter holder mounted onto the lens. Although it seems as though it may dislodge from the lens, I have never had any problems. Its quite stable even with one or two glass filters in place.
Here is the lens, filter and light sleeve, which I have found essential. I also have used the small cap to slide over view finder otherwise light will enter the body. But with these items in use I have done exposures up to six minutes with no light leakage.