The all rubber tape on seals over the edge of glass, can be inserted into a metal frame or channel and trim with a knife or razor blade.  It seals against dirt, moisture and weather, as well as providing insulation against sound and vibration.  Tape on seals is preferred for vent work because it turns tight corners easily, yet won’t stretch out of shape, and it’s thin enough so that it won’t crowd the glass to cause cracking or breaking.