Now here's a bird's eye of the holder's slots.
The idea is to shoot most of the time with just the two slots -- one for ND grad, the second for a second ND grad or a straight ND to get motion in fluids.
But in some instances, I want the option to use a CPL in front of that to (a) tame blue skies at longer FLs or (b) tame reflections or look into/thorugh water at any FL.
And for those who would say "I'd just screw on that ring when I need it" / "I'd likely go with one slot and add a second when the shot calls for it", please lose yourself of that notion. Assembling the 105mm ring and screwing it down is an exercise in patience, dexterity, and frustration. The Lee foundation kit screws (which I was stuck with, no new screws are offered with Lee's adaptor ring) are just barely long enough to catch, and screwing it down took spectacular pressure and perhaps 20 minutes of failed attempts before I got 3 out of 4 screws to take. One still spins freely right now no matter how hard I try. (I will be heading to a local hardware store for a replacement that is longer, but it is stable for now.)
The take home in assembling this is that you can make changes with a small screwdriver, but I'd never try to do this in the field. I would only take this apart with great light and a nice work surface.
Vignetting checks come next...