Yeah plans are IMO a great starting point - however you need to know what you intend to do and adjust those plans accordingly. Take for example my cab. The shape is nothing new, basically I took a look at a bunch of arcade machines, made some decisions and drew up a plan based loosely around several different types. I paid close attention to things that really matter in terms of playability - control panel height and angle, screen angle, and width. I then chose the size screen I wanted in there which determined the width of the cab, and then everything else was just style.
You have to make the width fairly close to the size of screen, otherwise you end up with a hideously wide bezel - same for height around the screen. That should also give you an idea about how deep to make it, because size = weight and real estate! No point in following the plans for a 24" cab, making it narrower for the 19" screen and leaving it 3 feet deep! Most people around here make a simple [and sometimes not so simple!] mock up in a 3d editor, the most popular free one being google sketchup.