Jessica, Chris, et al,
You can find retailers of UV filtering Lexan/Plexi-glass on the web. Acrilyte is one trade marked brand that can feature UV resistance. The thing to remember is that not all Lexan is UV resistant-you have to specify. 
Regardless of your chosen method of installation (e.g. a full panel to cover an entire window or a separate panel for each window pane), you probably want a pro to cut it for you. Therefore, I would suggest buying locally if possible. Check with a picture framer if you can't find it anywhere else. We measured the windows to determine an average size that would fit all, which kept the cutting price down. While moisture can build up between the Plexi-glass and the window pane, our panels are not airtight which has mitigated this problem. A spacer of some sort between glass and plastic to allow airflow is an option. Again, we used flexible points to hold the panels in and they are easy to remove for cleaning. You can get a 'point gun' from any large framing supplier.
The mixed reviews on whether or not to apply UV film directly onto old glass centers on the fact that the film has a life span and once it degrades you have to remove it. Typically done with a solvent, there is the potential for damaging paint, finishes, etc.. And, if a solvent doesn't get it, you have to scrape it-not a good idea on blown glass windows. Retailers of UV film used residentially will tell you that it is no problem to remove it, but I am skeptical. Modern glass is harder to break.
Using Lexan or Plexi-glass panels to filter UV light isn't a perfect solution, but it does work. I used a light meter to prove it and saw a drastic reduction in the UV light hitting our collections. Another idea that also cuts down on visible AND UV light damage-we have installed cloth roller blinds that have UV filtering in them. These allow a certain amount of visible light in the house even though we keep them drawn all the time. Hunter-Douglas is the manufacturer.
When all is said and done, I am still hoping that someone will invent a roller blind of clear, UV filtering plastic that is easy to maintain, replace, and wouldn't damage your windows.
Cheers,
Fielding
Redcliffe Plantation State Historic Site
Beech Island, South Carolina

 

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