how can I make a thin surface out of a closed spline?
Here is a quick tutorial for you.
Here is a closed spline sketched on the top plane.
With the sketch selected, choose the extruded boss/base feature.
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.)
enter the thickness of the thin feature and click the green check to approve.
the results (I am sure your guitar body will be better looking then this)