@tangled&tron good luck with doing the whole mod the X-acto blade. You're a brave soul than I. Having used these for decades, whenever I use them (even the little bit used in this project) they always seem to strike fear in me just a small bit! I'm afraid of dropping the thing and have it stab my foot or it slipping and slicing my finger to the bone! Though that's never happened (*knock* on wood) it could yet be a risky tool.
Now, the case is relatively thick and very hard as it's made for kids so they had to pass many drop tests, I'm sure. The cuts were difficult enough with a Dremel. If you can, buy a jewelers saw, like the one below, from a craft store or the like. They usually only run about $7-$15 and the blades are replaceable. Again, good luck and post a pic of your end result! Thanks.