I own my own art framing business and have made light boxes for various things. Attached are some images from a show at the Museum of New York. These boxes have a white tray lined with led strips inside, topped with a piece of white plexi. The artwork sits on top of that and then the frame slides over the whole thing. They aren't exactly cheap, but are really nice.

A custom frame with an open or closed back is pretty simple if you don't want a light source in there.

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These were $650 each in 2015. I could make them cheaper if someone wanted to install their own lights. It's all custom so it's a little different each time depending what the client wants.