Some of us have never used a router for these operations. Basically, a sharp blade and a spacer block. If you look up violin purfling cutters you'll get the general idea. It takes patience but with proper care ( always keep the cutter holder tangent to the body ) this operation (cutting a one-sided binding channel )"fails" into the waste. That is, if the blade wanders it goes into the waste from where it should be. A #11 Xacto blade almost works but it widens the cut line as it goes deeper. So, with a #11 you can cut a very shallow line then excavate the channel with a small, razor-sharp chisel. After that, the inboard line is defined by the initial cut and you can "whittle" it deeper. I did it this way for a long time.
Now I create my own cutters ... very thin, very sharp, and very short and flat on one side. The best approach if you want to perfect this method, is 1) build up a couple throw-away ukes, (sides and top) for practice or, 2) practice on a junker, and, 3) accept nothing but perfection. Don't give in to the idea that you can fix it with filler.
The cut needs to be very clean, but it can be done. Think of how many hundreds of thousands of violins have had their purfling done this way.
