Sorry, I’m not explaining this too well. Think of a through dado (face up as if you are chopping it out on the bench) that’s about 2″ deep by 5″ wide, but with the two bottom corner edges 1/4″ radii. I’m thinking the molding planes Dave mentioned that Lee Valley carries might do the trick if the little extra width on the plane doesn’t contact the side wall.
I’ve got a power router but am trying to resist the temptation to go back to power tools.
Thanks Gary for the sandpaper idea. That holds some promise.
Paul would probably somehow use a screw, a paperclip and chewing gum to do it in 3 seconds. ; ) I keep waiting for him to chime in with an elegantly simple solution.