How to cut into plastic to remove a metal slug that had been press-fit to be flush with the surface?
If you can cut it with an xacto you can cut it with a razor blade utility knife (aka carpet knife, Stanley knife), like this:



The blade is thicker and more durable than an xacto, and the handle allows you to apply more power to the blade. 

My option 1 would actually be a rotary cutting tool like a Dremel with a cutting wheel attached, however it's an expensive thing for a single job, the utility knife will do the trick for much less money.