What Lee cpl do you recommend? The 105mm round one or the 100mm square one?
the square lee CP that's 4x4": http://www.bhphotovideo.com/c/product/216637-REG/LEE_Filters_PLC_G_4x4_Circular_Polarizer_Glass.html
the square one goes in one of your slots in the normal holder. Lee foundation kits come with 2 slots "installed" and a third in the package. I've put the third onto my holder and the slot closest to the camera is always used for the Lee CP unless I'm doing 10-stop shots, then I swap it for the 10stopper. The Lee CP is glass though, and I've had one slide all the way through that slot when moving to a new location, so either pull it out when you move or add 2 screws to the bottom of your holder to avoid this:
here's my 2 added screws:
we also have a thread just about the Lee square CP here: https://photography-on-the.net …hp?t=883069&highlight=lee
and I also went through the whole CP with NDs thing here: https://photography-on-the.net/forum/showthread.php?t=998032
I've since shot with a buddy who's got the 105mm CP on the front of his slotted holder. He gets vignetting on his 24-70 with that, we swapped holders for a bit one day and it's somewhere in the 20s on a 17-40 where you see the corners go black. That setup simply does not work for landscaping IMHO. I wouldn't bother with the double lee holder for the square one either as you'll inevitably run into that same problem. The issue you have using the setup shown above is that your CP is locked into the same angle as your GND. But in practice it's not an issue at all. No you don't have perfect polarization all the time, but it's been good enough and much better than the other options in terms of how you use it.
and don't worry about asking questions. That's what the forum is about and we've all had to learn it somehow.