Any tricks or tips for making narrow slits in hard foam strike indicators

I have a bunch of Lightning Strike hard foam indicators that I would like to convert to slip on style. Are there any tricks for making a very thin slit in hard polystyrene foam? I tried a coping blade and an electrified wire and both make cuts that are too wide.

How about heating a razor blade with a torch and melting it in?

or,

x-acto makes a pretty thin saw blade for their stuff, it is thinner than a coping saw blade.