Hi Gang,
I was thicknessing my headstock with a (upydowny sander), oscillating spindle sander and got a little carried away and have sanded the headstock about 2mm too narrow for my tuners, grrrr! Before I add a veneer to increase the thickness, does anybody have any suggestions 🤔 cheers in advance 👍
Can't be sure from looking at the angle of the photos, but it looks like the back of the headstock needs the add-on rather than the front?
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Bummer!
If I should do it: Let the veneer go all the way to the end of the fretboard and make the groove for the nut later. I would roughly make a cut out for the truss rod adjustment before gluing it on.

Yes you are right. I didn't look close enough. A bit trickier to get a good fit against the volute area, then.
I'd glue a plate on the back, 3 or 4mm thick, make it a feature. Maybe add a bit of shape where it joins the neck, make it look on purpose.
With this one I cut to deep when cutting out the neck, I thought oh well, cut another 2 lines to make a triangle then filled with ebony dust and superglue lol
I've routered a floyd cavity on the wrong place and all sorts but everything has worked out great in the end!
I see yours is a scarf joint? So no real need to do a big volute, that's why I'd add the wood to the back instead then shape into a small volute. A nice contrasting wood. Or maybe a nice figured wood?
You could glue on a piece 6, 7 or 8mm thick then thickness it down leaving a nicely carved volute. Carved from the piece you added.