Make a copy of the channel you just masked out and begin shading it to match the light source you've established in your picture. Apply a Gaussian Blur of a few pixels when you have it shaded the way you like it.

Copy this channel (select all, COPY) and paste it into it's own document. (ctrl+N , ctrl+V). Make sure the new document's mode is set to "Grayscale"

Flatten the new document and save it as "displacement.psd"

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