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