Bill, I am learning a LOT from you and your videos. Appreciate the time and trouble that it takes.
I will give YOU a tip, tho, with the patch box cavity - use a Forstner bit for the excavation. They come in all sorts of sizes, increments of 1/16". And I wouldn't doubt that you could get metric sizes also. And you already know how to set your depth on the drill press or milling machine. That way you only have to chisel the "V" shapes between the round holes.
If you can find some overhead pics of old patch boxes, a lot of them you will see three (+/-) small divots left from the builder using a drill bit done with a brace. Sure does eliminate finding you went a bit too deep with one or more little holes.
I am anxious to see your finished engraving, IF you can pry it out of your wife's hands!