Monofilm is a light weight, inexpensive mylar material that will not stretch. It makes good windows in sails because it is relatively light and very clear. The major sailcloth used in windsurfer sails. Also used for boardsails and telltale windows.
This reinforced X-ply monofilm (a clear polyester film with a black thread running through it) is another choice for windows in sails. It can also be used for windsurfer sails. It is very light in weight 4.2oz, yet quite stable and durable. Sold per Yard.