This polarizing filter is the best choice for your scanning rig.
It's a high quality 3D printed frame together with a high quality polarizing film.
The filter is attachable through magnets directly to the ringflash and stays stable in every situation. Thanks to its magnets and the handle it's adjustable in 45 degrees steps to easily change from cross polarized to parallel polarized within seconds. It stays cool even on long scanning trips because of it's ventilation holes on the side.

This polarizing filter is compatible to Walimex Pro HS-400, Godox Witstro AR 400, Quadralite RX400, Flashpoint RF-400 and RIKO400.