There is a way to get a CPL for (nearly) free. All you need are 3D glasses from the cinema (usually disposable articles). If you are not passionated in watching movies (like me) you might have to ask a fried (my friends have “thousands” of them at home).
Detail note: Some cinemas are using other technologies than Polarized 3D system and won’t work therefore.
Follow these (trivial) steps to get your CPL:
- Detach one of the lenses of the 3D glasses.
- Place it in front of your (mobile phone) camera.
- Aim a window (with a lot of reflections).
- Rotate the lens (of the 3d glasses) in front of your camera until you get the best result (sometimes you get better results when you turn the lens round).
- stick the lens with tape (or unicorn stickers
)
This is what it looks like on my phone (I’m shure you can do it much better (e. g. cut out a part of the lens, use “real” tape):
This is what my dashbooard looks like:
And here a few shots for comapring (first image: without 3d glasses, second: with 3d glasses, photos where shot by mapillary app):
As mentioned in the wikipedia article the color of vegetation appears a bit different:
As you can see: Also the reflections of the window of the other car are reduced. 
Imho the results are awesome (especially for a costless solution).
greetings
descilla