Generally speaking, the Impervious power modifier is traditionally applied to the Protection power, or potentially straight to a characters base Toughness value if it's high enough to not need the bonus Toughness from the Protection power. However, the modifier just says it applies to any save, though traditionally Toughness, to others with GM approval. With how Impervious can be applied to Create Object, does it seem feasible to be able to add the Impervious modifier to a Snare effect, given appropriate descriptors, since it too has a Toughness save?

I wouldn't stop anyone from doing that if they want their snare to be more durable.

As a note, you can also apply Impervious to Fortitude and Reflex (see Sensory Shield for example) and Will (see Mind Shield).

I see an Impervious Snare being either something made of super-hard material, or something incredibly gummy that is difficult to damage. Either way, it forces people to use strength to break free, or Escape Artist to slip out, rather than just beating on it. It is a good thing to surprise a player with if they always do things one way.