Bill, Microscale 'Krystal Klear' is basically refined PVA adhesive (white, wood-working glue), packaged and sold for making small windows, or for gluing canopies in place etc.
Put a small 'blob' on the tip of a cocktail stick, and place this on one edge of an aperture, then draw it across the aperture, until a 'skin' forms. Wipe off any excess with a wet finger tip and, once the KK dries, it leaves a clear 'window'.
It can also be used for making light lenses, beacon domes and so on, by 'extruding it to a peak which, when it dries and shrinks back slightly, will be dome-shaped. If, for example, you wanted a red anti-collision beacon, then paint the base red, the make the dome with KK, and when set, it will look like a beacon.
A good quality PVA will do exactly the same, at a fraction of the cost of KK.