I removed all of my existing duct work that was designed as a radial system with a sheet metal rectangular plenum with very long flex runs poorly attached. I am replacing my system with all round sheet metal duct as an extended plenum design. Assuming proper duct sizing calculations, I am wondering if it is reasonable to attach a flat piece of sheet metal to the air handler to make the connection to the round duct instead of installing a new sheet metal rectangular plenum first to connect the supply round sheet metal duct.
Resources: Duct System Design Considerations
Manual: Model: chpf3636b6ca
This picture illustrates what the end result would look like: A round duct directly connected to a rectangular air handler. This picture is not of my existing system.