Just curious what do you shoot with?

I would suggest IKEA or any furniture stores that are throwing out those huge boxes. Recycle, help the environment, pick one of those boxes up. They can be collapsible and just tilt box over and slide your poster board in. But paper create glares, so try with felt/fabric and adhere to the inside of the box. And of course use energy efficient bulbs.

My approach would be taking my shots around early morning where there is no cloud or too bright sun that causes shadows.

But if i’m press for time, any good digital camera has very impressive near colors quality with flash on. if it’s not bright enough in cheaper quality digital camera. Take your image as for instance your last darker pics, use Paint Shop Pro and in two steps, use the brightness contrast to brighten the image, then “fill in” tool as “white background” and you can achieve your result in less than 3-5 seconds.