If you could email me your email address that will accept an attachment, I will send a couple pics I have taken of the two circle compasses we have purchased over the years and you could easily fab your own. First pic is of the circle compass for the manual oxy/fuel torch and second is a pic of the circle compass for the manual plasma torch. They are simple to set up and use.
1) Set the compass to the radius you desire and place the torch head in the holder.
2) You still have to adjust your cutting speed manually, but it guides your hand in a perfect circle.
John Wright
to answer your question of whether a plasma will cut the thin material you have is, "Yes". HVAC guys use cnc plasma tables to cut out the ductwork, they make all of their own fittings from flat sheets.