You can use butcher paper underneath and around to camouflage the junk in the background as is the case in this photo of a water fountain . Around the fountain is fake ivy, which also helps to creat a scene. In this photo of a dragon , it was sitting on a table with a sheet of Uroboros behind it. If you are using a camera with a light meter, take your meter reading right at the panel, then shut the meter off and back up to take your photo. Because the lightbox tends to eliminate things like texture in the glass, it's hard to tell that the background is white confetti glass, but if you look closely you can see it.
Cloudy days are the best for shooting installed windows, especially if they face South.
Never take the photo with the sun directly behind the window. If it faces West, shoot in the morning. The easiest ones to photograph face North. If you have to shoot a panel that has a lot of transparent glass and don't want the junk in the background, try holding a big piece of butcher paper behind it. This one was photographed when the sun was almost directly behind the window.