I was going to suggest a stronger alternative like heavy duty aluminium tape. We called it speed tape at work. That was because the adhesive withstood the drag on aircraft travelling at over 500mph. It's weatherproof. I think the equivalent can be found in the public domain where roofers buy their supplies. I didn't mention in the end since it can't be soldered for electrical bonding.
For strength, should you have any concerns about brittle walnut down the line, I'd sacrifice a further half millimetre thickness from the top and replace it with a piece of 26 gauge (Imperial) stainless steel sheet cut to fit snugly without adhesive. The pot and switch attachments should stabilise the assembly. I'd recommend against bonding anything thicker than tape to the walnut because of differential expansion due to changes in temperature. You can still use copper tape if required.
Nice to see some form emerging. Looking good.