After experimenting a bit with my router and a scrap piece of the 1/4" BB ply, I found that even with a minimal depression cut into the wood, the prongs of the torque washer extended through the other side once tapped in. So I tried simply tapping it into the surface of the ply without any depression, and the photo shows the result.
I suppose I could try glassing over that, but I am also wondering if I need that torque washer at all. Would a simple square hole, carefully drilled and filed through one of the cheeks (ply + 2 layers of glass on each side) be sufficient?
Greenlake, did you embed a squared off washer of some sort or just go with the square hole idea?