I did a show in Italy under the bright mediterranean sun this summer and one of the locations we had was this hair salon with about 12x6 ornate picture windows. We shot in the location every sunday (the only time we could get it) for the four weeks of shooting in Genova. We were shooting with the dvx-100 and obviously could only handle about two or three stops over before becoming completely blown out. And during the day, the sun was hitting the buildings opposite at an f64 at noon, and changing about four stops during the day. So I had to apply a sheet of nd6 and and a sheet of nd9 to each window, and adjust them during the day. Doing that to 4 windows over 4 seperate days was an absolute pain...but I learned something about gelling windows!
The best way I found to gel was to constantly adjust the gel, smoothing it out, applying and reapplying tape strips. If you do it right, it'll stay forever. It just takes a lot of time and a lot of patience.
I hear there's a patented gel cutter out there somewhere...not a bad idea.