I give here an idea since there is no such place in Poppy head. I thought about a backpack fixed on “chest” component. In this backpack can be a large plate covered of smooth Velcro. You can fix anythink you want on this plate with Velcro.
I am a Velcro fan 
Good idea, maybe you can turn this suggestion as a call for contributions ?
I tried the backpack to store electronics. There is a major issue with this solution : the center of mass is shifted backward and the robot is not centered. To stand-up, the robot has to lean forward which apply constant load on ankle motors.