Egg crate is really lousy as a tank cover. It does nothing to slow down evaporation; it's also much more abrasive than a smooth glass or poly top, especially if it has any cut edges exposed to the fish.
I've seen some folks stretch netting across the top of ponds, securing it around the edges so that if a fish jumps the net just stretches and then rebounds. It's soft so will probably cause the least damage possible. Like eggcrate, it won't help with evaporation, but will be much easier on the fish.