I'm pretty sure there are no current flies tied specifically to imitate the Water Penny. I really don't know if they are a significant food source to trout. If I was to tie one I would just use a pair of those round Velcro tabs about 1/2" in diameter and put one tab on the bottom of the hook shank and add some glue and put the other half on top and press them together so the Velcro hook and loops mesh and the glue is distributed well. After the glue dried I could color the top and bottom with various waterproof pens and trim the diameter to match the size of the natural.
Lately I've become interested in water pennies, though I can't say why. Is a water penny nymph fly in my future?
Anyway, I took this picture today. It is a live specimen that I collected from Trout Creek, which flows into the Cannonsville Reservoir / West Branch of the Delaware river in New York in October 2013. It's current home is a 2 gallon aquarium it shares with some common stoneflies. I used a 50mm macro lens on a Cannon EOS camera with brown felt as the background. I made a special "V" shaped micro aquarium for it so that it would attach to the side wall and not the bottom so that I could get this picture.