cross section from hull - techniques?

That type of scribe method is painful. It only works if the scribe is perpendicular to the work, throughout the scribing process, which is nearly impossible to achieve on curved surfaces or tracings. It also requires you have a tracing board nearly the size of the thing you're trying to scribe.

 

We have not felt any pain with this method.

The first rough scribe and cut when the ply is above the 2 x 4, gets the curve easily within 3/16. Then when it dropped into the hull you just redo the curve with a compass, ( we use a log scribe)

The first rough scribe used the 1 x 3 with the ball point pen above the corner of the 1 x 3, as the ply pattern is going to be moved DOWN from the top

The second scribe will deal with moving the ply from the right to the left and the pencil in the compass and the point will be held horizontal

Now that the gap is small, the precision is not so dependent on the angle on the compass. A 10 degree variation off horizontal assuming a 1/2 inch spread will only change the horizontal distance about .008 inches (cos 10 degrees X.5 inches)

For the vertical rough scribe, and a height of 36 inches of depth, a 10 degree variance will only result in a 1/2 inch error

We use this method to shape our stringers and it is extremely precise and fast

 

That will give you one side at a time, you'll have to figure out a clever way to line the two sides up with each other so your bulkhead fits the opening properly.

If that fails, try a joggle stick, it works very well and costs nothing. It works anywhere you need to fit something to a shape or space and will do straight edges with sharp corners, which that flexible thing won't do.

 

I've used those mechanical spines and they don't make particularly fair curves. At least the one I've used didn't. It made curves, but they weren't fair.

 

I forgot to mention that the log scribe has a built in bubble level system to ensure that it is held either vertically or horizontally for the scribe.
This tool is invaluable for profiling furniture, stringers, bulkheads when building a boat.
Lee Valley Tools, Veritas