How would you go about the following mortising job if done with a router?

Let’s say I have long, curved legs that I will be putting on jewelry box, and for sake of argument let’s say the legs have the shape of a “wishbone”, with each vertical element of the wishbone being one leg. Now if I wanted to join these legs with a horizontal piece approx. 2/3 of the way down their length, via a loose tenon, I will need to cut mortises in each of the legs. How does one go about routing a mortise in this situation as there are no right angles with which to work?