Important Update: I did this conversion for a friend, and we used the smaller compartments on the red clam-shell as attachment points (watch the video to see those compartments). They appear to be just as strong, and I actually prefer the way the keytar hangs better than my original design, And you can use a $6 tube of epoxy to fill the compartments, instead of the $40 box.
https://youtu.be/MP-uA7TGR-0