All you need is a cardboard box, some white fabric or tissue paper, a box cutter and a piece of posterboard. My box measures12.5"*12.5"*19". It seems to be the perfect size for taking pics of cards, but you can use what you have on hand. Depending on what you are photographing, you may want your box to be smaller or bigger.
First, measure in one inch from the edges of the top and the sides of your box. Cut a window on the top and sides, leaving that 1" border. It doesn't have to be perfect. I'm not good with a box cutter, so I just did my best. (dh watched me like a hawk, waiting for the blood to start spurting, but I insisted on doing it myself!)
Next, cut a piece of white fabric or tissue paper to cover the openings. I used white flannel because I had it on hand.
If you use tissue paper, you may have to layer it to get the right light diffusion. I used hot glue to attach it to the outside of the box. Pull it tight. It shouldn't warp the box, but you don't want it so loose that it casts shadows.Cut your posterboard hot dog style to fit the box. You want a gentle curve at the bottom back of the box, not a crease. This will give you a seamless look. I used masking tape to attach my posterboard, only at the tippy top.