Here some notes of CPL filter.
My dvr is GS1000 and i found out that there is two sizes that are easiest to install, 25mm and 30,5mm.
I used cheap (16 eur) Marumi CPL filter because it has those sizes available.
Soon i noticed that CPL filter is must have accessorie with dvr, it removes all glare that comes from your dashboard to windshield.
However, experience taught me that cheap cpl filter is not flawless, these cheap filters give nasty reflection when sun hits it from the side, that is because it's made of cheaper materials and it's not intended to work in direct sunshine.
Then next thing to do is make some kind of lens hood or install camera deeper inside your car so sun don't hit it directly.
Other possibility is buy more expensive filter which work better in the sunlight (maybe 80+$) but then you probably can't have small filters sizes, i'm guessing that smallest reflection free filter is maybe 42mm or so.
All of these filters are made of two sides of glass and one or more plastic film is pressed between them.
It means that you can't cut filter to smaller bits, because if you do then air will go inside layers and destroys it in X amount of time.
My advice is.
-Use cpl filter.
-Shade it or install dvr deeper in the car out of direct sunlight.
-Buy expensive reflection free cpl filter and step up rings to make it fit.
These filters are made of glass, if you take good care of it it will last the rest of your life, so in that perspective price is not so very important factor.
Here you can download sample images of dashboard glare on/off, and tube video.
https://mega.co.nz/#!xwcESTTR!a7jvHz_dz ... FChCf1479k
http://youtu.be/UP21F7OT7rg