Last December I started thinking about joining TechShop, so at Hanukkah I told my girlfriend that if I joined I could make her all kinds of dreidels. Today I found myself with some extra aluminum, so I decided to use their metal shop make my first one.
The barstock I had was 1"x3" 6061, so I first needed to make a 1"x1"x3" rectangle. Measure the thickness of the bar now; you'll want to match it exactly when you make the square. In my case the bar was 1.016" thick, and that's the size I wanted to match in the other dimension.
We're going to face the bar stock to make sure it's both the right size and perfectly square. However, it's easier to be precise when you have a the whole bar to clamp in the vice, so I'd recommend facing one end of the bar before you cut it. This is an extruded bar, and the sides are remarkably square, so the only sides I need to worry about are the ones I (or the factory) cut.
Carefully measure and cut a little wide so you have material to trim off. I used the horizontal band saw.
Finally, stick an end mill in a milling machine and get the remaining side nice and square at your desired thickness (1.016" in my case). Be careful! If you take off to much you'll have to rotate it and introduce more likelihood of un-squareness.