Fender neck on a Gibson set neck body

[dave.king1]

I have used overlaid masking tape to create a pattern for shaping wooden cover plates. You lay it over the void, run a fingernail or a skewer around the edges to mark them and then remove and trim to those marked edges. Lay that over your plug blank and cut to rough size, then test fit and sand to final size.

That's how we used to make gaskets back in the day, and still do on occasion