You really don't even need a box at all. You can do it, with just white seamless and 3 lights. First off, do you or do you have any flash systems at all? Or are you just using standard lights as in the two pictures above?
Also the difference between the two pics above are the bottom one is using two pieces of white translucent pieces of cloth which allow the lights to go through them, thus "evenly" spreading the light and softening it.
The top photo, they are using just whiteboard and bouncing the light off the sides. That is why they are having to run the two lights through the front.