I haven't seen this mentioned here, so just to note that Cokin make a holder for their filters that screws into the tripod mount of the camera:
http://www.cokin.com/ico15-B.html.
I have an older version with only two "rails" for the tripod screw to go into and it just about fits in front of the lens on the GRD and doesn't seem to cause any vignetting on the 28mm lens. The deeper mounting shoe on the version illustrated would probably fit with the 40mm tele-convertor, though I'm sure it would vignette the 21 mm. Useful for mounting ND filters, for a start. And I have a whole pile of silly ones I picked up really cheap second hand ages ago!
http://www.cokin.com/ico15-B.html.
I have an older version with only two "rails" for the tripod screw to go into and it just about fits in front of the lens on the GRD and doesn't seem to cause any vignetting on the 28mm lens. The deeper mounting shoe on the version illustrated would probably fit with the 40mm tele-convertor, though I'm sure it would vignette the 21 mm. Useful for mounting ND filters, for a start. And I have a whole pile of silly ones I picked up really cheap second hand ages ago!