Here are some details about the F-3H Demon fuselage plug assembly.
The Fuselage plug inner core was milled with matching alignment holes. These holes allow the insertion of M10 threaded rods that serve two purposes: aligning the two halves and pressing them together at the gluing stage.
Before gluing, the halves are aligned and measurements are taken to verify the width of the plug. It is not uncommon that milling deviations create an offset in the parting plan, thickening the two halves . This is easily detected by verifying the plug width once assembled. Here are a few pictures of the plug after gluing.

Here is a view from the front that shows the alignment of the seam line.



The same view front the back, that shows the alignment of the beaver tail flat surfaces.



And a side view of the nose area. The threaded rod that presses the front halves is visible here, as well as the flatness of the canopy surface.