how can I make a thin surface out of a closed spline?


Here is a quick tutorial for you.
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Step 1:
Here is a closed spline sketched on the top plane.
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Step 2:
With the sketch selected, choose the extruded boss/base feature.
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Step 3:
Place a check in the Thin Feature option and choose mid-plane.
(I like this option and it centers your thin feature on the spline, the other options choose what side of the spline to make your feature on.)
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Step 4:
enter the thickness of the thin feature and click the green check to approve.
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Step 5:
the results (I am sure your guitar body will be better looking then this)