I placed mine on the metal brackets/core support right above them (I've got a 2006/LSE bumper), and this worked perfectly for me - I have the smaller 35W digital ballasts.
Used 3M heavy-duty double-sided trim/moulding tape to secure it.
I have one mounted on the bottom of the windshield wiper resevior and the other mounted on the inside of the bumper towards the drivers side tire. Over 2 years and no problems with them there. Doubled sided tape and then a strip of duct tape over the ballast also.