Would "είσαι σε καλή μορφή" work? If so, it's currently marked as incorrect, general strengthening on web. As is "είσαι σε καλό σχήμα".
I would say that they both sound awkward, and they are not commonly used. "Είσαι σε καλή φόρμα" would definitely work, though. You hear and see that phrase quite often :P