Mike, you have mail.
I started to make one like MarkM did, using the cake pan, but I figured since I had access to a CNC laser machine and Mechanical Desktop (souped up Autocad) that I would just design and make the parts myself. There is no reason that you couldn't cut the parts with a good set of tin snips though.
Honestly, if you could find a cake pan with the right diameter, that would be easier. The way I did it was pretty difficult and I swore that I would never do it again, but, it fits perfectly and its pretty fun telling people at shows and at autocross events that I made that air cleaner. Another reason that I didn't go with a cake pan is because most of them are coated with something or they are stainless steel. I wanted more of a stockish looking air cleaner so I had to have something that paint would stick to.