>I know that I can't use silicone to glue them in, because it
>will deteriorate in sunlight. Any advice on a long-lasting glue
>which would decently withstand sunlight and also the minimal
>expansion and contraction of the window sills?
There are 30-year-rated silicone caulks available at most hardware stores.
It's been years since I used them professionally, so I don't have any
recommendations. Some can be applied to wet surfaces.