How can I transition a square duct to round of the same outer size?
They do exist. Best bet is probably to contact a local sheet metal shop, if they don't have one they can build one. Expect this to cost in the tens of dollars range.





You could potentially cap the end, and then use a round duct takeoff. The downside is it will restrict flow to the vent because of the abrupt end and sharp angles internally.





There's another type of side takeoff that is rectangular, and if you can find one you could potentially modify it yourself using a couple strategic cuts and copious amounts of foil tape.