Ok, here's the second picture, a close-up of the bottom. In a private e-mail to Scruffy, I mentioned to him that the magnetic sign material was great most of the time, and prevented the cup from sliding on the fender, but it wasn't strong enough to hold the cup down when I was bush-hogging an old corn-field. So Scruffy promptly responded by telling me I needed an old hard drive magnet. The next thing I know, he's sent me some of them! (Thanks, Scruffy!)
What you're looking at in this picture is a ring of black magnetic sign material glued to the bottom of the cup and one of the hard drive magnets glued into the depression in the center. Originally, I covered the whole bottom with the sign material, but when that wasn't enough, I cut the part covering covering the depression in the center out with scissors (thus the ragged edge) and glued the hard drive magnet into it. I used Marine Goop for both jobs. (I love that stuff!) These magnets are so strong that I didn't really want them in contact with the fender, so I didn't build it up flush, but left it 1/8" or so below the surface of the sign material ringing the outer edge. There's room for another magnet, but I can't imagine needing it, because it'll hold the cup to the side of the refrigerator, even when full of water! Yet, because it doesn't touch the fender, it's still easy to pick up, as long as you tilt it first, instead of just trying to pick it straight up.
MarkC