If you don’t have a football-shaped cookie cutter, make a pattern out of waxed paper and cut around the pattern with a knife, or cut freehand.
Instead of football shapes, you can cut the bars into squares. Before cutting, frost the bars with ganache and let stand until set. Then cut into 4 rows by 3 rows (for 12 large bars) or 6 rows by 4 rows (for 24 small bars).
If the ganache softens, put bars back in the refrigerator to firm up.