Duct bracing to stop flexing

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?
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John wrote:

I use light weight angle iron.
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Joseph Meehan

Dia duit
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I was thinking S-cleat. But then, heating guys like me work with cleat.
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Christopher A. Young
You can\'t shout down a troll.
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Stormin Mormon wrote:

Yea, everyone tends to work with what they know. If I were a heating pro, I guess I would use what was most convenient and what I had in the truck . Me I work with what I have in my garage. I guess I should have been a little more clear. Us what works and what you have to stiffen it.

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Joseph Meehan

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