Well, I'll be damned - I got a lens with a rear filter holder. Thanks for sharing
I did just fine with cloud blur using a screw-in B&W ND3.0 77mm on my 17-40L. I got 30s and longer exposures with small apertures and low ISO.
Try the Marumi 16.5 stop ND filter.
I recently realized my 17-40L has this back filter holder :-p and I now even have though of a use for it: With my 10x glass screw-in nd filter I don't get long enough shutter times for cloud blur in bright daylight, so I'd like to add another gel nd filter.Question: What nd gels do I get to put behind the 17-40L?