If you own an unmounted 138mm polarizing filter, you can use this Rota-Pola Tray from Revar Cine to install it into a matte box that accepts 4 x 5.65" filters. The tray has a lightweight and rigid aluminum construction, and rotating the gears that extend from either side of the tray will adjust the polarizer in order to easily achieve your desired result. The advantage of this setup over a standard filter is apparent when the matte box is mounted on LWS rods. This setup usually prevents the full 360° rotation of 4 x 5.65 filters, but with the Rota-Pola solution, the filter rotation takes place inside the tray, so there is no interference with LWS rods. Additionally, matte boxes that don't feature rotating filter stages will now be useable in shots that require a rotating polarizer.