You may have seen these in the Make Blog. This is how to make your own.
Step 1: The Finished Speaker Box
This is the finished article. In this case the box is an 80mm cube but it could be any size you like just choose the card thickness to suite the size of the box.
Although this was made from card you could just as well use polypropylene as a harder wearing material, you can often buy stationary folders made from polypropylene that you can recycle.