Mike, Phils suggestion will work like a champ, as long as you can firmly clamp a straight edge to the face frame. I've done similar things multiple times though yours is in an awkward location and getting dust in your eyes is an issue. I'd use a pattern bit with a top mounted bearing. If you don't have one, MLCS should have one and the shipping used to be free.
You probably realize this, but there is some chance of hitting a staple, pin, nail or screw somewhere along that face frame. Check it closely and keep clear of flying carbide.
BTW, after routing the bulk of the material, leave the straight edge clamped in place and it will do a great job of guiding a hand saw to finish the two ends. If you cut carefully, it will turn out nearly as well as the actual routed portion.