Hey Vellum. If you are going to do it in PS I would start with a large template as you can always reduce the size later to fit on your map. That size really depends on how big your maps tend to be. As for creating a template... well, you could do that with the pen tool. Or just draw one side, duplicate that layer, flip horizontally and position it.
I'll try to throw together a few screenshots for you here in a minute.