Natural light is great. But a lightbox is designed to mimic natural window light. Sometimes they're even better than natural light, if the only light you have is very very dark, or you only have access to direct harsh light from the sun.
If your material is thin enough, you can try taking your lightbox outside in direct sunlight, and use the lightbox to diffuse that harsh light. I might try that as an experiment when I get home! Thank you for the inspiration :)