Where do I get this part for an oil furnace?

In a way, maybe I should wait and see what happens tomorrow, but I am
hoping to learn something in advance.

I just noticed or just created a hole in a sheet metal part of my oil
furnace, a small box that connects the firebox to the flue. It looks
rectangular from the front, top, and bottom, and trapezoidal from the
side**, shorter at the front than the back. It is open in the back to
connect to the firebox and has a round hole near the front in the top
to connect to the flue. (Is there a name for this part?)

The man is already scheduled and coming to clean the furnace late
tomorrow morning. Even if he leaves his shop without the part, if his
company stocks it, I can go there and get it while he is at his
earlier customers. Do they stock things like this? The Carrier oil
furnace is 20+ years old. If I give the company the furnace model
number, might they have this part in stock? Or do they make one when
they need one? Or will I have to find someone to make me one?

Is he going to insist I need a whole new furnace because this one part
is crumbling? If he said that, would he be right? I ran it for
about 20 minutes last month and it worked fine. The hole appeared
today, probably after I pushed on the metal with the shop vac tube.


**I have more exact measurements but it's roughly 5" high near the
firebox, 2.5 inches at the front, and 18 inches deep, and 14 inches
wide. Plus it has a flange around it that gets screwed to the
furnace.