The challenge with a filter in front of a sensor is, that it has to meet high requirements:
- optically exactly parallel polished
- bubble and inclusion free
- streak and striae free
This very hard to meet since off the shelf filter glass does not provide that usually, as it does not matter in front of a lens - but in front of a sensor (i.e. about in focus), all that gets visible in your image.
A good converter checks his filter glass under a microscope, locates the needed size which fulfills the a.m. requirements and then cuts that small out from a larger piece.
Personally I would rather buy such a filter precut from a professional camera converter, as he would be liable for the quality and not try risky experiments since "cheap is way to expensive" in the end.An example borrowed, showing a bubble in SCHOTT IR filter glass:
Example for striae in glass:
Stay tuned, more will follow on that fascinating subject...
More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos