The Abyssus didn't work for you?
Check out the CM "Xornet II". It's a really strange shape and very short and I remember I thought the mouse was unusually flat. I don't know if it's still sold.
You could try to find a really small mouse so that only fingertip grip works. There are a clones of the "Logitech Mini" shape from Chinese and Korean brands, but I don't know what names to search for and where to get them.