So your last CPL fell out the bottom of the holder? I just got the 4x4 CPL last week and have been using the 105 previously. I got the 4x4 because I don't like the vignetting below 14mm on my 10-22 while using the 105 CPL. So I am guessing the filter holder will wear in some more and the 4x4 will not be held quite so firmly?
It happens rather quickly, too! Early on there wasn't much of a problem, but now I can slide the CPL in, while sitting staionary in a tripod, gravity will be enough to slide the filter out and onto the floor. I'd recommend edging the filter with some electrical tape or something similar. That does the trick.
Ben, your idea was similar to something I wish Lee would offer.
There's some money waiting for someone to design something like that.