Nice! The same thick plastic windows from the Small Paper Theatre Box - Clear Window Insert (linked below), but in larger sheets. Perfect to make a window to cover the front of a room box. Or create stained glass windows with markers, or create faux glass over a portrait like in Laura's artwork shown below.
Sold individually (1 sheet, with a thin protective covering to protect from scratches; I'm holding the thin covering in the photo).
Georgian Ladies Parlor by Laura Carson; See the Supply List.
More info in the blog.