Something like this https://www.metalsdepot.com/aluminum-products/aluminum-channel-6063? 1/16″ is perhaps overkill for this type of application—but then perhaps having it provide some structural strength is a good thing.
The other option would be make a drawing and then go talk with a local HVAC company to see if they make their own custom duct work or if they have a supplier they will introduce you to.
Something like this https://www.metalsdepot.com/aluminum-products/aluminum-channel-6063? 1/16″ is perhaps overkill for this type of application—but then perhaps having it provide some structural strength is a good thing.
The other option would be make a drawing and then go talk with a local HVAC company to see if they make their own custom duct work or if they have a supplier they will introduce you to.