Sort of a tool....but before you cut anything make sure you have the body all squared up, well supported, and some braces tacked in so that the body itself remains straight and square when the top is removed....

As for tools, my trusty sawzall is my favorite tool....best thing is of course to have your cuts well planned out rather then just hacking away.... Templates made from posterboard also work very nice to make your cuts the same from side to side......