How to create a indicator on top of each entity using A-Frame for VR?

I am using A-Frame for VR and I am looking for a way to pin a second entity on the top of all other A-Frame entities like the black diamond in the image below:

I have 2 main challenges:

• place automatically the diamond: I am able to place it relatively to its parent as a child but I need to manage myself the size of the parent to be sure the child is not inside the parent
• rotated the diamong in order to get it perpendicular to the user view at every moment.

To be honest, I would like to recreate a indicator visible on top of each entity on the screen like in watch dog 2 for example (in the example the diamond is white on the top of the car).

I don't see any easy to do it with A-Frame and I would like to know if I need to code it by myself or if there is something that could help me.

Thank you in advance