Yes, you have to custom make it.

The inside edge will pretty much follow the end of the inner stem, and then as you shape it to match the hull it is quite intuitive.

Uncle Nick has a good video on making it:

https://www.youtube.com/watch?v=-V0l2aOb_Us&index=34&list=PL5A97438DE4A600BD

Or, you can make one out of laminated thin strips, this is much easier, although you do need the very thin strips to laminate:

https://www.youtube.com/watch?v=WBHiPAazlaI&index=59&list=PL5A97438DE4A600BD