I was finishing up an archtop neck and would like to show a jig that works well for cutting the notch for the extension and gluing the support piece. It could be used for a supported extension in a modern style flat top as well.
The jig keys off the truss rod slot. It serves as a router guide for the notch and a gluing/clamping jig.
Clamped into place with notch depth set
Routing the notch with a flush cut top bearing bit.
Notch cut.
Support block slides into place. Spring clamps keep it aligned.
Clamping setup.
You have to have your neck set and alignment done and all mounting hardware installed before doing this.