A curved bottomed spokeshave is good tool for the job. Another would be a compass plane. As you are working an end grain concave curve you'll find it best to work from the centre of the board/ panel/ cabinet or table top towards the outside corners, thus working as much as you can 'with the grain'. The chattering may be caused by a variety of things: trying to do too deep a cut, a less than firm grip, and a blunt blade all can contribute, and if you skew the tool a bit that should help. Having said that, curved bottom spokeshaves aren't the easiest tools to use, and you can help them a bit by using a bit of candlewax on the sole, as well as making sure the other points I mentioned are covered. You might also find you could use a drawknife pretty effectively for this job, especially if the wood isn't overly dry. Slainte.