I've had this idea in my head since I got the original Japanese release, and I finally got around to doing it. I figure since his tail pieces join up with his wings in alt mode, they could attach there. Then they just have to fold in half and rotate to sit nicely on his back in 'bot mode. As you can see I kept it simple with a thin piece of speaker wire serving as a flexible joint. Two closely-spaced ball joints would do the same thing if you felt adventurous. Alt mode is not changed in appearance at all. The only other thing I had to change was the nubs near his fists. They had to go to make transformation possible.