Best material for shrinking holes in the boat (i.e. installing portlights)

Since direct replacements for the large portlights on older Ericsons do not seem to be available, has anyone glassed a material around the perimeter to fit something smaller? What material did you use, and how did you do the glass so it does not look shabby when you paint over it? My whole hull needs repainting, so I have some freedom on how much glassing to do.

Gareth
Freyja E35 #241 1972