Got a bit confused with getting the Universe and Target the right way around. Looks like you can do either. But only the Targets get passed to the unmatched output node. So if you wanted to see unmatched customers, make sure they are the target.
Here is the solution that I showed at the Grand Prix.
At Inspire, each driver solved this one a different way - one with the Find Nearest tool, one with a Trade Area and Spatial Match tool, and one with an Append Fields (to create all possible combinations), Distance, and Filter tool.
Don't forget to register for the European Grand Prix prelims!