There are lots of ways to do this, all of the easiest involve some kind
of router jig.  You can buy a commercial product for cutting ellipses
at, for example,  http://www.microfence.com/pages/Ellipse%20Jig.html and
others. (Google 'router oval jig' or 'router ellipse jig' to find quite
a few options).  Highland hardware provides free plans to make your own:
http://www.highlandwoodworking.com/library/infiniteovaljig.pdf

That said, if I had only one to cut I would make a template out of 1/4"
plywood and cut freehand using a either a guide bushing on the router
or a template bit with a ball bearing guide.  The advantage to using
the guide bushing on the router is that you can make a decorative cut
if you like.  Keep in mind that the front of your piano is most likely
plywood and you're not going to like the raw edge -- a molding will
probably required that you can make using the same sort of jigs or
setups in a concentric fashion.

To make an oval template you can use the classic method of two nails
and a string or a variety of other ways -- for example, see
http://www.astro.cornell.edu/academics/courses/astro201/ellipse.htm --
or just use the drawing tool on your computer to make the shape you
want on paper, attach it to the wood and cut it out with a jig saw, and
smooth the edges if needed with file or sandpaper.

You can use either the outside ring or the center cutout as the
template, you just need to size it appropriately to make the hole the
right size.  I think using the outside ring is a little safer -- if
your router wanders you're cutting into the waste piece rather than the
part you want to keep.  (You don't need to ask how I came to this
realization!)  A practice cut in scrap is suggested.

If you want a different sort of shape, like a racetrack oval, you can
make your template with circular or ellipsoid ends and a flat top and
bottom.

Cheers,
Roger Wiegand
www.band-organ.com