I used a cove box bit on my plunge router and made a jig from mdf. the bit was pretty hard to find beyond amazon because it needs a bearing on the top of the shank. here's the link
then in order to sand I used a dremel attachment in my drill from rockler, cant find it online but this is pretty close
I recommend using a micrometer as well, it only takes like a 1/10th of an inch for it took look very off center.