I incubate mine at about 77. I'm not sure if 70 isn't too low for them. Definitely don't put heat pad under box with eggs. It will most likely overheat them. You can try giving the box with eggs into drawer and placing the heat pad few inches from it. I've done that several times with no issues, during winter, when my room is generally colder. Eggs hatched after 7-8 months. Just don't let the temperatures in the drawer hit more than 84.