In this instructable, I will show how I used Autodesk 123D (free) to design a part.
What I do is "start with a box" and remove everything that is not a wire guide.
Follow along--you'll see that it's easy.
Design files available here:
In the highlighted box, type the number you want then press the "tab" key to move to the other box. Click when you are done with a step.
Click on this--it defines a rectangle.