Not really "Boondocking", but J.W. Wells State Park between Escanaba and Menominee on Green Bay has 3 really nice "primitive" or "rustic" sites by the lake. I don't think they're reservable, you just show up, should not be a problem after Labor Day, especially during the week.
The park also has electric only sites along the lake in the "Modern" campground, but it can get crowded during summer and weekends.
Wells State Park Detail