I just used a hack saw or an electric saber saw or jig saw if you have one. by the way yoou can tie the egg crate pieces together with those plastic sinch tie things they sell at home depot. They work great.
Diagonal cutting wire cutters (dykes" tin snips, hack saw, band saw, scroll saw, jig saw, or just a hacksaw balde alone does a number on egg crate. Scissors do not work well, too long of blades and too flimsey.