When board's assembly is complicated, or somthing is being soldered in pcb banded position we do metal templates.We also do flat metal templates when we can not use PCB frame (when PCB is so big that all is left is technological fabricators frame) or when heat sink is needed for soldering.
BUT most of times you should do PCB frame.
You see, when PCB is being made it is bigger than your pcb, This is cold panel . Panel can contain single PCB or multiple PCBs, and they can be any kind of PCBs. PANEL contains PCB it self, test coupons and technological frame. All of it is made together. Your PCB in soime stage will be milled or cut out from panel. and some times PCB is holding to FRAMe with necks.
What you need to do is create panel with your PCB (assembly panel) within fabrication panel.
https://static.electronicsweekly.com...el-example.jpg (
https://www.electronicsweekly.com/bl...-part-2014-02/)
https://icape.libellab.eu/flexrigid2/ - nice demo
Ho[e it will help.
I usually draw assembly panel in mechanical layer and make Notes in FabNotes layer, frame layer, board shape layer and in PCBs drawing (documentation). Note says "PWB should be dispatched within assembly panel. Panel drawing data should be extracted from *Name of gerber*" ,you also should specify do you want a V-cut (and what kind of it of do you want neckdowns. Your assenbly house should tell you by what means they can get out your PCB from Panel. Of course, It would be better is you do fabrication and assembly in same place, so locals do the works and not you.