Stopping Down NP101

I need to stop down the NP101 to F15 or so to cut back on the amount of light coming through for solar viewing. I am using a Cromixsun rear Halpha filter setup with four filters. To go to 3 or 2 filters lets in more light and it is recommended that I stop down to F15 - F20 aperature. I figure I need a front lens cover with a 35mm hole in it. I may also be able to use a nuetral density filter I need to contact Marcello Lugili the maker of the Cromix sun. Thanks for any comments - Dean