On Tuesday at Queen Creek Maker Night we made awesome light boxes. Everyone had a lot of fun making them. The cool part is you don't even need colored lights to make it fun, just hold it up to anything with light and it looks great.
To make these you mainly need mylar. In theory you could use old mylar balloons but you'd need a lot of them. I estimate 3.75 square feet (3 inch wide pieces totaling 15 feet).
We used vellum as the diffuser since I had some and it looked great, but regular printer paper also looks good. Either way you can get both mylar and vellum online.
The box frame was made with foam board, we used cling wrap to hold the parts inside. Just a bit of hot glue and clear tape to hold things together. The final touch was adding scrapbook paper on the outside to cover up the tape and give a nicer look.