Very nice blog! And thanks for the review. About the fit of the insert-- if you wedge something between the body and the catch tabs to give them more force against the opening lip, they'll grip the lip better. Some fit tighter than others, and I test-fit each one and adjusted the catch tabs to snap nicely into place in my car, but in some cars a difference of a few microns can change that.
One of the first mods I did on my GSW was to swap out US rear headrests for Euro headrests. Since my car has cloth seats, I was able to find matching ones used on UK eBay for cheap. The view out my rear window is now completely unobstructed whereas with the US headrests the view was at least 20% obstructed, even with the headrests all the way down.