Improving the crystal tester’s (nonexistent) grounding requires a band of copper tape around the inside of the proto board holder. Rather than cut the tape lengthwise to fit the holder, a new one will be just tall enough:
While I was at it, I deleted the washer recesses, because those didn’t work out well, and fiddled the screw holes to put the inserts in from the bottom:
Although the overhang inside the holes will be ugly, I’ll epoxy the inserts flush with the bottom and nobody will ever know.
The copper tape now makes a tidy ground strap:
With a gap in the front to eliminate the obvious loop:
The OpenSCAD source code as a GitHub Gist: