Eddie, not sure but I think the best combination might be a cheap standard thickness filter to sacrifice as a blank spacer and then use a slim filter for the NDs, I'm not convinced that two standard thickness filters won't vignette but I might be wrong. Interested to see how this works out.
Id have to re-read the FM thread but I think they used two standards and didnt get vignette. I dont even own the lens but just posted it here for the folks that do




