A soon to be registered member, Blair, also known as the King of Cardboard, built one for our room at Marscon a few months ago. As with most of his props, it's mostly cardboard. I'll let him explain when his registration is finalized.
I forgot to mention: it also moves. Blair built a neat little rocker cam setup so that it would slowly scan back and forth. We had looped sound effects from the battle, and of the scanner, so it was pretty neat.
I had been looking at last years Marscon photos and found this. It was our Command Post party room with a War of the Worlds theme. The Matian Probe can be seen in the upper left corner. The flash of the camera has washed out the lighting effect on the probe. As OB10 said, we had sound effects going on as well.