DIY Photo Light Box Tutorial:
White posterboard (should be at least 30% longer than the box length)
Cut off the tops of the box.
Pop out the side
It should look like this after you cover all three sides.
Cut white poster board the width of the box.
Place the poster board in the box such that it curves inside the box.
Cropped with a little brightening and voila!
Here is another side-by-side to compare. Before: