How do I make a light box faux stained glass window?

Lowe`s has stain glass fireplace shields for in front of your fireplace you buy one and take apart there are three pains of stained glass then glue or silicone to your window and there you have what you want without the mega cost ....

Here is one I just finished. WE happen to love southwestern also. I hope this helps. https://www.hometalk.com/diy/decorate/rooms/faux-stained-glass-quick-and-easy-34976564

Or buy from someone, who makes stained glass windows

If you can find old Windows in the size you need, you could used stained glass paint on them, using permanent market as the leading line, or buy frames ,poster frames or try thrift store, either use plastic sheet in them or get plexiglass to fit and use glass paints or you can go to Amazon and put window film in the search bar and you'll see a side assortment . Post when you are done, sounds great. I use old Windows N put them over my Windows using both techniques that way easy change without having to bother the permanent window. Love them.