First of all, the concept is just awesome and really looks incredible on that wood.
For your border, I’m thinking you might want to apply this technique
but modify it slightly. Instead of just taking the image and making it black, first put a nice thick stroke around it then flatten that and then make it all black. That should allow you to make a contoured border that could be more to your liking.
Same thing happened to me the other day. I had it under a bright light and thought I’d gotten all of the masking off. Took a pic to show it off and saw quite a bit more!