Someone posted a thread that got resurrected a couple weeks ago but I'm at a loss to find it that shows the Lee filter system attachment rings & brackets. In that thread one respondent mentioned the use of step-up rings to get to the largest size needed for the 'attachment ring' to screw into. As I have 67, 72, 77, and 82 filter sizes to deal with as well, I've gone the step-up ring method for my CPL. When (no specific plans yet) I want to start with ND and GNDs, I'm going to get the 82mm size 'adapter' and simply 'step up ring' as needed. Plan B would be to have a different Lee adapter for each size, but that can get pricier than I can handle.
The only problem with using step-up rings to use larger size filters than the lens uses is that the hoods no longer fit. I've resorted to using my left hand as a quicky-hood when needed. Some have used thin flexible cardboard in these situations with good results as well.