A tiny wire was passed through the hole, and soldered to the copper inside the trench.
https://m.g3l.org/system/media_attachments/files/112/861/691/183/493/403/original/04a94024bb16303a.jpg
Also, I noticed the pad for the negative battery terminal was so gone that the pad left on the other side didn't connect anymore to ground. I had to dig the epoxy to find the ground plane, and solder a wire to it.
Can you spot the wire on the last photo?
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