Trick I learned from one of my model RR friends (HO, but I overlook that); I have used
this method to make lenses, diesel porthole glass and windows. Using a glue called "Sobo"
(from Hobby Lobby; I am sure that others will do - I believe that it is for fabrics)
and a toothpick, twirl a glob of the white glue in the opening until it grabs on all sides,
all the way across, then leave it alone for a day. Leave it in a position where gravity pulls
down on it from the proper direction to avoid sagging while it is wet. It will dry/cure clear, and is surprisingly tough. You can add more to the dried lens if you think that it is too thin; let cure again. They usually look good, are cheap and easy, and can be easily repaired, though I have never had one fail. Typical incandescent headlight bulbs don't seem to faze it.
Sobo is also a pretty good glue, in general; I think that it is water-free, so it doesn't
cause warpage of paper, cardboard, etc.