This is similar to what Toughman suggested. I also removed all extra green dummies so that there are no blocks leftover. Take a look at it and see if you like it. If not I can switch it back and you can work on it some more.
although possible now... i have to say that there are way too many dummies. I'd suggest your first approach with one small change (i think it would be possible). One of the outer 4 greens (or better the two opposite in order to be symmetrical) to be placed 1 square to the inside (that is the top right green 1 square to the left and bottom left green 1 square to the right). I believe you would increase the challenge and make a good use of the extra dummies. Anyway it is your call.