Barry Irby
Duct work parts are your friend. Use Google and find an elaborate duct supply catalog. Look up "boots" and "take offs". Take offs are designed to fit on the side of a trunk duct and then connect to a (usually) round duct. They can be 4" or 6" or something else. Boots go on the termination end of the duct and transition from round to rectangular. A huge variety of configurations are available. Sometimes the rectangle is at ninety degrees to the duct and sometimes it lines up with it. I found four inch to 2 1/4 by 10" end boots to be very useful. You need access to the catalog and to a large supplier because there may be fifty different variations manufactured but Lowe's and the like only carry abut 10% of the them. I cobbled together my system and it ain't purty, but it works pretty well and was cheap.

Here's a picture of an End Boot on my bandsaw.

And a take off on my jointer, worked perfectly without any modification to either the jointer or the takeoff.