To make the shelves, I used two boxes of approximately the same width. The longer box was used for the main body of the shelves. The shorter box, I cut into shelves and supports. Since the shorter box was the same width, I just cut it into four strips. One strip was placed at the bottom and top of the longer box to give it more strength. The other two were placed in the middle to make shelves. For the shelves, I left part of the sides on the strip along with the piece I had cut from the bottom of the box. The pieces of the sides pointed either downward or inward, providing support for the shelves.
Three or four layers of strong cardboard is surprisingly sturdy. Each shelf and the supports were screwed into place with half inch screws. First, I taped them in with packing tape. This gave them enough stability so that they didn't move around when I screwed them. They also were not as hard to hold in place. Some of the screws could be screwed in all the way. The others only most of the way. It took about 27 screws to finish the box. This seems like a lot, but a bag of screws only costs a few cents. The boxes were recycled from something we had ordered in the mail. All in all, it probably cost about $1.00 to complete the box.
The shelves can be left as is or painted or covered with pieces of wrapping paper. Decorating will increase the price of the shelves to between $1.50 and $2.00. Since it will be in an out of the way place that nobody sees very much, I might just leave it as is. Haven't quite decided yet. Anyway, here is what the unpainted box looks like. I messed up a little on the top shelf -- didn't quite get it cut evenly; but it still works. :-)