So I ordered some sheet aluminum from onlinemetals.com. This place is nice because the sell sheet, bar, tube, etc. metal goods in retail quantities. I ordered a 12x24 inch sheet of .0125" 6061 aluminum. While I was waiting on it, I made a full-size template of the panel, with the locations for the jacks and mounting holes, as well as a switch. When the metal came in, I used this to mark the length and width of the piece that I wanted. I bought a metal cutting blade for my circular saw, and had it.
The results, as you can see, were not terribly satisfactory:
The printed template is show here next to cut metal. As you can see, the cut metal isn't a consistent width; it's wider in the middle. I found it difficult to cut a straight line with the circular saw because I had to go so slowly, and because the guide on my saw isn't that good. The burrs were also really bad -- it was like the saw melted some of the edge and it built up an irregular edge on both sides. I worked on it with a Dremel armed with a diamond-grinding bit, and that got a lot of that cleaned up, but eventually melted metal clogged up the bit. Nonetheless, I think I'll proceed with drilling and assembly and see how it goes. I haven't decided yet what to do for a finish.
Here is the template overlaying the metal: