Is the diameter really critical?
Yes. I am looking specifically for a pot that will fit down INSIDE this firebox and set on top of it's "firesticks" (little support rods).
http://foldingfirebox.com/ The walls of the firebox will come up surrounding the bottom of the pot. This is for when I'm using the firebox as a potholder/windscreen for a Trangia burner.
For sitting ON TOP of the firebox, anything will do.