Hi Andrew, If you have the "Patience of Job"
you can cut clear acetate to fit the inside dimensions of the upper and lower shashes of the Grandt Line windows. A very minute amount of a clear drying glue in each corner will hold them in place.
Down and Dirty Method
... Both MicroScale and Micromark make a "liquid glass" material. Using a tooth pick you fill in each glass pane and when it dries its clear and simulates glass...Don't use too much...keep it as thin as possible or it may dry cloudy. If you can't find these products go to a craft store and get some SOBO Glue. It works just as well.
Wanna Tourture Yourself Method
... This is for those who wish to really beat themselves up
Obtain a box of large microscope glass slide covers. They are real glass about half the thickness of a piece of paper. Using a diamond or carbide tipped scriber, scribe and break the glass to fit the inside dimensions of the upper and lower sashes and fasten them in with a minute amount of SOBO glue in the corners. Be sure to clean the glass with a Q-Tip and alcohol before you install it and then only handle it by the edges so you don't have fingerprints on it. I only did this once but the results were incredible.