There isn't really a lot you can do. Cortisol is a stress response in the body from internal or external stress. No supplement is going to blunt that response.
Making sure you are taking adequate anti-oxidants will help. I would supplement with a good quality multivitamin like ruthless 3d from n2bm.com each day.
Drinking green tea throughout the day will also help lower cortisol levels. The polyphenols will also raise dopamine levels which are your body's feel good hormones. That will help reduce stress in your daily life.