Ron,
You are correct on the size of the quarter round, and I just glued them in place with some light clamping, no nails or screws anywhere in these speakers and they are solid as a rock. I did not use a roundover bit, I hand sanded the quarter round edges where needed to get them flush, and very carefully used a belt sander to round off the corners.
Thanks for the compliment on my work! I have done a few other small projects around my house the last few years with oak veneered plywood and oak trim on the edges. I did a mission style audio gear rack last winter that turned out pretty well, so my confidence has been building. I had enough wood on hand, so I thought, why not try it on speakers?