Hi Pete,
I have a 70-200mm f4L lens on my 40D with a 67mm diameter - I use a 72mm Hutech filter with a 67-72mm step up ring ($9 on ebay!!)
I would avoid the clip in front filter (even if you could use one!!) , I had heaps of trouble with the one I had...
http://www.iceinspace.com.au/forum/s...ad.php?t=35552
I am very happy with the configuration I have now
Doug