The International Cessna^® 170 Association

I don't think that they often use aluminum for firewalls, do they? With a relatively low melting point, I'd be reluctant to use it for patching one

You may have a point on the aluminum BL. I guess it might depend what and where I was patching.

In my case, it's to make the round heat vent hole fit the square heat valve.

Galvalume is available from many metal bldg suppliers.