AIf you're using the support from the bag behind the makeup you could use either method. Depending on the size you could make it out of Gumpaste and skip the cardboard.
I would avoid the toothpicks if you can, or make sure you tell someone theyre in there. Either way you'd want to make the holes before it hardens.
No the eye shadow panel will not be supported by the bag. I am not making the bag, it will be free standing. I was thinking that gumpaste would be heavy and it would fall down.