You might try making a sheet music figure out of gumpaste that is rolled up and then you could place the neck resting on it. I made a guitar cake all out of cake but cheated a little and had the same height for the neck as the rest of the guitar so supporting the neck wasn't an issue.
Good luck!
I also kept the neck the same height as the rest of the cake. I covered the top with the frets and then the lower part, where the hollow of the neck would be I just covered in black fondant and covered the cake board in black too. It's in my photos if you want to see. hth
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