The substitute rubber seal is a good idea. But do people have experience
with installing reproduction vent glass? It seems that the "gap" problem
is mentioned in several postings I have seen, and the repro glass I bought
(2 different vendors) seems to be almost impossible to seat in the frame
and avoid having the gap. I understand that you don't need to jam the glass
all the way into the frame, but in order to eliminate the gap, it seems
like the glass needs to be almost out of the frame.

Or maybe the problem is the repro rubber that rivets on the frame - is it
too short, and so doesn't meet the glass?