Yes you use sheet metal screws to attach plenum. It should also be a good fit. I purchased a plenum at Menards 50 bucks maybe other home centers will carry them. These plenums you then cut to fit. Leave a little room for adding these. S-Cleats. They come in a long stick that you cut to fit.
They'll add a little support and beef up the sheet metal a little. That top cleat on that page is for fitting a run of two square ducts together. This shows what the plenum will look like. You'll need to cut to fit.
Also used those s-cleats to attach inlet side.
Not familiar with AC coils. Good luck. That working with sheet metal
was the only hard part of installation. Wear some good gloves when working with sheet metal.