I searched for some good spider images that I could convert into a silhouette but found none. I did find lots of good reference images though.
I fired up Google Sketchup, as it can work as great and easy line drawing program with a mirror function. It also allowed me to layout my frosted glass panel to scale and visualize how large I wanted the spider to be on the glass.
I drew a cool looking spider, and a few minutes later I had a to-scale print out to use as a guide to cut my cardboard.
The template I made is online here: http://sketchup.google.com/3dwarehouse/details?mid=f25f707c99ef38358b62e185294e8b2e