Hi
Long tome ago I tried the same project on an Italian virginal.
It took me 15 min per whole. I tried it once just with chisels, the
better way would be to make a punch for the long sides of the whole. The
short sides can be cut with a chisel.
Drill with smaller drillbits (or use the router),  than cut away almost
to the line  and than use the punch for the exact size.
 
Good luck and best Regards
Thomas Glueck
Clavichord Builder
URL: http://members.aon.at/glueck.clavichord