I used a 2" Astronomik CLS filter that screws into the camera adapter. It worked great with my 5DmkII.
I just happen to be selling it at the moment as I'm shooting narrowband with a CCD now and don't image with my 5DmkII anymore. Check out this thread if you're interested:
http://www.iceinspace.com.au/forum/s...d.php?t=108736