Pentax DFA 21mm and screw in Lee ND filters

A word of caution if you own a Pentax DFA 21mm lens and wish to use a screw in ND filter. I purchased a Lee Elements 67mm little stopper for this purpose but discovered that the filter ring has two knurled sections which are quite chunky. They are probably a good idea for normal use but on this lens it makes the filter ring too wide to fit inside the hood!
Filter being returned.
Does anyone have recommendations for other makes which actually fit inside the hood.
thanks
Paul

Sorry Paul - I can't offer an option I did email Lee asking about the possibility of an adaptor for the SW150 system - they said there were no current plans, but if demand was there they may consider it. So if anyone wants to badger them we may eventually see such an item

There are other brands that produce ND screw filters in 67mm. When researching this topic myself recently, I came to understand that Hoya Pro ND 1000 and Kenko Real Pro ND 1000 screw in filters are good alternatives to the Lee Elements Big Stopper. B + W and Heliopan also produce good 10 stop screw in filters. I have no idea how thick the filter rings are on any of these brands’ ND filters, but I would guess that they are thinner than the Elements Big Stopper.

Then there are all the Chinese filter brands, NISI, Haida, etc, that I unfortunately know very little about, but many are probably equally good compared to the European brands.