Halloween // Advice on severed head prop

So this halloween for my costume I need to make a severed head prop that's pretty lightweight as I want to carry it around with me during the night. I was thinking of doing a life cast of my friend's head cause it'd be fun for the head to have a recognizable face but I don't know what material to make the actual prop out of. Plaster is a no go, it's too breakable and too heavy. Does anyone have advice on materials for the head?