Basically, make notches so that .... umm.... might need to draw it.
Ok here's the drawing:
The first picture is the front view, when you're looking right at it.
The other pictures are how you construct it.
Each "X" is two pieces of (cardboard, plywood, whatever) that have notches cut in the middle, a bit more than the width of the material (like if it's 1/4 inch thick, make the notch more like 1/2 inch), exactly halfway through.
Then rotate them and join the notches.
Then connect them as shown.
You could make something perfectly usable and nice looking for super-cheap with cardboard and the right duct tape.