The main return air duct mounted next to my upflow furnace flexes inward
when the furnace is operating. After the blower shuts off, the sheet
metal pops back out (oil canning) with an annoying bang sound. It looks
like a brace was created by the installer halfway up. It needs another
brace or bracket between this brace and the humidifier above it so the
sheet metal can't flex.

Where can I find such a brace? Is something ready made for this
purpose?