I saw a recipe where they cut the rind completely off a large piece of watermelon the size of a cake and frost it with homemade whipping cream and berries!
No bake is a wonderful idea -- the cake is an alternative to my frosted angel food cake as there is no cake in the watermelon cake, the inside is pure watermelon.
It looks wonderful, but I think I would prefer a nonfat frosting, like Betty Crocker's Fluffy Frosting, which is composed of primarily egg white and sugar. It is a light and fluffy frosting.
The white frosting is decorated with blueberries and raspberries for a red, white, and blue, July 4th design!
Details on how to cut at http://www.isavea2z.com/no-bake-watermelon-cake-recipe/
I wonder how this cake would be half watermelon on the bottom and half angel food, for a really high cake!