The best deal is 4 x 4 filter holders that take 3 x 3 filters for $2 each. These are Century brand that were $24 each last year. 3 x 3 filters are much cheaper and more available. These holders fit all brands of matteboxes that take standard 4 x 4 filters.
Here's the deal, folks: I called them and they have lots of the filter holders in stock. I doubled my order. 75mm wide (3 x 3) filters are good for 95% of my lenses.

These are good quality holders/adapters & are a gift at $2 ea. I bought 24. Plus -They can be modified for larger (or smaller) filters to fit into standard 4 x 4 matte holders.

There are also many 3 x 3 schneider filters at $10 that were $138. I bought all the ND filters but there were dozens of others (low contrast etc.) left. You can't have too many filters !

The other good deals are 77mm adapter & filter step-up rings.
I spent almost $300 and got $2K worth of stuff. Good deals!

@bubba Glad you were able to find some deals!

@tonalt

Step-Rings won't work. The Series-9 has no threads.
They fit into adapters that DO have threads.

Some 4x4 Matteboxes will hold a series 9 filter, via a filter holder adapter.

The $2 deal on 3x3 filter holders (that bubba mentions above) also hold Series 9 Filters.
It is a 4x4 filter holder that adapts 3x3 or Series 9 Filters to fit in a 4x4 MatteBox(Not all Matteboxes are compatible)

https://www.schneideroptics.com/Ecommerce/CatalogItemDetail.aspx?CID=1760&IID=1780
note: this adapter does not rotate, so you'd need to pull the adapter out and rotate the polarizer manually.


Or, find a cheap Series 9 filter/adapter combo via ebay or vintage camera shop.
Best to find an adapter that rotates. Like special effect (3 and 4 Star) filter w/ rotating adapter.
Make sure the adapter includes a retaining ring.

Series Filter info per wikipedia:

Starting in the 1930s, filters were also made in a sizing system known as a series mount. The filters themselves were round pieces of glass (or occasionally other materials) with no threads. Very early filters had no rims around the glass, but the more common later production filters had the glass mounted in metal rims. To mount the filters on a camera, the filter was placed between two rings; the mount ring either screwed into the lens threads or was slipped over the lens barrel and the retaining ring screws into the mounting ring to hold the filter in place. The series designations are generally written as Roman numerals, I to IX, though there are a few sizes not written that way, such as Series 4.5 and Series 5.5. Most Series filter sizes are now obsolete, production having ceased by the late 1970's. However, Series 9 became a standard of the motion picture industry and Series 9 filters are still produced and sold today, particularly for professional motion picture cinematography.[13]

Nice idea, name it the 2ugly9'er.

Buy a cheap ebay polarizer of your size choice, knock out the glass, and glue.

A 2ugly9'er on a spit. :-)

Sorry about bad quality picture but there you have it, Series 9 filter attached to my old Minolta without mattebox. I drilled a circle to the filter holder with dremel, put 105mm-to-77mm filter step up ring to it and used also some tape. Better solution would be to use somekind of clamps to attach step up ring to filter holder but I haven't found proper ones.

The polarization filter turns in the filter holder.

From 77mm I use another step up ring to fit with different lenses.

The problem with this system is that it's hard to control in which way the filter holder will be when it is screwed to end. You cannot turn 3x3 filters when them are attached to the filter holder.

Question: Does it matter in which way 3x3 filters (e.g. white frost) are, or does them have to be always in 90 degree angle?