...in my opinion eraser-material is way to soft for that job!

Take an old worried out Flanged Magnum hull, put one drop of hot lead into an empty (fired!) primer cup (sanse the anvil), seat that special cup into the primer pocket with a drop of good epoxy - here you are with ready-made snappers for your Flanged Magnum NE!

You can also use bakelith material for the primer section instead of the "leaded cup"......