Greetings. I need a geometry expert, please.

I have an 8 inch, 390 degree, chimney pipe coming into my basement at a 45 degree angle. I need to cut out a tin heat shield that lays flat against the wall, around the pipe. Obviously, it would be "eliptical-ish" (my word), but I wonder what the equation for this would be?

Thanks very much
Dave Vanwerkhoven