Passing an already-instantiated reporter CFC to testbox throws an error when you try to run the tests. That's because the logic to test and see if the reporter is a struct also fires for CFC instances. See this complete example including error message on Gist:
https://gist.github.com/jamiejackson/98236d89e47f8e2bcced9d0234537a3c
Making the two if statements in TestBox's produceReport() function for isObject() and isStruct() an if/else should fix the issue since if the reporter is an object, we don't want to check it for a struct as well.