What I'm trying to do is a contour cut on a guitar neck, but without cutting the headstock off as it was shaped in another process.

So think of it as a contour around a four sided box but I only want to cut three sides, like a U shape, avoiding the headstock.

Problem I'm facing is the contour command needs a closed profile, unless there is something I'm missing? I've tried setting the three sides as a contain but it doesn't like not having a closed path.

I tried a constant step over 3 axis move but cannot get that to work either.

I've redrawn the neck as a simple block, a 2d sketch and extruded straight up with no detail, as I couldn't figure out how to do the contour on the 3D shaped model to start with as it's a collection of extrusions, not one extrusion.

How would you approach this?

Thankyou