How can I make a sunken electrical outlet line up with an outlet cover?
If you use a spacer the inset must be equal to or less than 1/4 of an inch to be NEC compliant.

See this question for relevant information: How do I extend outlets after installing a backsplash?

Personally, I would just get a box extender like the one in the answer to the question above.  They are fairly cheap and better by design (in my opinion).