[Three Peaks]

A jigsaw usually works great and you can get different grades of blade depending on how smooth you want to make your cut. For protection of your finish, I usually put down at least one layer of heavy tape for the foot of the jigsaw to ride on while you are cutting, then peal it off when you're done.

Rotary tools that look like a Roto Zip would work, but they are hard to control and get a clean and straight cut with.