Thank you for your many suggestions. I went lo-tech:
Sorry, this might have been more suitable to art-crafts forum than electronics.
1. went to Kinko's... they printed my picture on 8.5x11 transparency. It was a picture of a red-ish flower. (sample photo attached)
2. From large hardware store, I bought fluorescent light (about 5 in. long) that runs on 4 AA batteries. I used cheapo batteries bought in bulk. I was not allowed to use electrical outlet.
3. Instead of frosted glass, went with a sheet of white printer paper. I put the white paper behind the transparency. One sheet seemed not enough and 2 sheets too much.
4. With still too much light, I taped a small piece of colored paper to the front of the light.
5. It was still too bright, so I taped colored paper to the inside back of the shadow box, which was white. The paper had similar color to the subject of the picture.
6. The shadow box was about three inches deep.
It looked great, as the venue was not well lit and my picture stood out among others.
The only regret was that the cheapo batteries wore out after about 4 hours. I should have used longer lasting batteries