Here's the thing: If your intent is purely home theater, then an exact match isn't a big issue. surrounds are used mainly for background and ambience sounds and the occasional gunshot or whatever.
Music, however does sort of require that they match the front three They get more usage on a regular basis and blending with the fronts is a bit more critical.
Since your fronts are custom made, nobody can say they have heard them in order to valid suggestions.