1) Find a PC, you know (library, school, work ...the one you sitting on right now) ...just kidding
Mine is steel but aluminum would work much better if you can find one.
2) Pop PC case cover open, and take it home it can be used in better ways.
3) OPTIONAL clean the paint off with sandpaper. It may stink and not holdup to hot temperatures.
4) Take a cardboard box and do the measurements
5) Trace them onto the steel sheet
6) Drill along the lines, file and bend the steel sheet.
7) My drill battery died....so next time folks more photos