I don't have a digital camera that will get in close enough to show one of these filters. Let me describe it a different way. On the magnetic side of the filter, you will have a strip down the center of the filter with no magnet. On either side of that will be opposing half-moon shaped magnets. Or, saying it another way: Cover one side of the filter with a magnet and remove a strip down the center.An added note since my first post - I bought more sticky magnet sheets for more filters, but the magnet material was too thin. It seems that it needs to be at least 1/8" thick to make contact with the steel around the lens.
I too will be making some ND filters, in my case for portraits. I have found with the lens set to near focus, WIDE OPEN and subjects about 12 feet away, the background goes beautifully soft.