I had started the hollowing of the back, as with the front, and then got down to the point of using my drill press to map out the basic graduation scheme. I had just seen a photo of another fellow doing the same thing, except with about a 1/4 inch drill-bit. I always had used a 1/16 or 3/32. Had a 1/8th bit in place, so left it there.
Here's my basic set-up.
Going around the edge, I felt the stop slip, and the drill just broke through the outside. It's the hole at about 10:30, with light showing through. The hole at the top, almost out of the photo, is the registration pin hole, and meant to be there. I think the thinner drill bit requires less downward pressure, and is therefore less likely to cause the stop setting to slip.
So, my first thought was to fill the hole with a dowel. Then I started thinking (a dangerous thing, really) that I wanted wood that matched better, particularly the direction of the grain. A dowel would have the grain perpendicular to the grain of the back. So I evacuated a roughly oval shaped area, found a piece of maple that had been cut-off from this back, and started to shape it to fit. The four little blocks are glued around the hole to make sure I put the patch down exactly the same way each time.
After spending way too much time fitting this patch, using chalk dust to see where it touched and where it didn't, I finally got to the point that I thought it might work. Fired up the glue pot and used fresh, thick hide glue.
Here's the area with blocks removed and the overhead of the patch removed. You can see the outline of the patch, but it appears solid. I didn't worry about matching the grain exactly -- the outside hole is about the diameter of a pencil lead -- but at least it's the same wood.
Actually, in retrospect, I might have been just as well to use a dowel.
With all that nonsense out of the way, I am now back to getting the top to its proper thickness.