Hi. Just pickup an end boot (5" or 6") diameter from the hardware supplier for that size. If you don't have a drive bender or know how to, simply use another couple of S cleats (cut them about 1/2" to 1" shorter) and slide the boot into the cleats. Drill 1/8 hole where you can in the middle of the drive in both ends or sides which ever you can (4 is plenty) then use a no. 8 or so metal screw or wood screw will work. After its fastened, take some metal tape and wrap it. Connect to the round duct and your done.
hope that helps. Marcel.