Routing the ends of narrow boards is awkward to do safely, with a good chance of a poor result, so Rockler's Rail Coping Sled is the answer. It can hold up to 127mm wide material up to 31mm thick, safely held between a fixed fence, and adjustable fence fitted with abrasive sandpaper for extra grip, under a tool free adjustable toggle clamp.
The 3/8" phenolic base is resistant to flexing under the pressure from the toggle clamps. The toggle clamps give ample holding strength when adjusted correctly.
Two handles gives control while keeping hands well away from the cutter.