An oscillating multi-tool is best, I think, as it allows a high degree of precision, plunge cutting, great depth control, doesn't tend to melt plastic (as would a cutting blade in an angle grinder) and is just dang nice to have around the shop. Things like cutting the rectangular hole for a new rocker switch become a piece-of-cake.
Buy one . . . you won't be sorry. I've standardized on Makita for all my battery powered tools, so they can all use the same style battery, but there are other good brands as well.