An old boat builders trick
Router works great attached to a rotating piece of plywood, as someone mentioned. If you don't have those tools try an old trick. Create a template out of plywood or whatever material you have that's cheap. Measure you're arch height and width and transfer those measurements on to your template material. Add an inch on one side of your width on one side. Cut the angle from the peak of the arch at the middle to the tip of the width on the side that does not have an inch added to it. Make a notch in the middle at the top of the angle to put your pencil in.
The template is made...but to make it easier to understand - check the photo