Polarizing Filter holder for Relio². It has an in-place joint to make it rotatable.
You also need a linearly-polarized film of `~30x30mm` size.
It's really
...さらに表示cheap and easily available con common online marketplaces like Ebay.
Material advice: PETG (Relio² reaches 60ºC).
http://www.relio.it
