You might be able to use the planters that are made for handrails on decks. Since the pond is made from wood, they might just sit on there perfectly. Some Iris, marsh marigold, pickrel weed, and Arrow Heads will fill in nicely. They also grow pretty fast so any brousing might not be noticed. I don't know if you can have floaters (hyacinth/lettuce) in your state, but if you can a couple of them for the Koi and turtle they will high five you. My turtle doesn't really eat the lettuce, but he like to hang out in them and chew them. He will eat pieces of hyacinth though.