I think the product is "Krystal Kleer". It comes in a small plastic bottle and looks like thick white woodworking glue.
I've used it for a lot of things. It makes window panes up to about 3/16" square. Run a line around the inside edge of the hole and then spread a blob across the opening like a soap bubble. Let dry.
Result is not perfect, but looks reasonable. Can't see through it well enough to tell if the inside is detailed.