defroster duct work

The a/c box had additional plastic 'Y' piece that changed a single square opening to 2 rounds.
Standard heat/defrost box has what you describe.
Detroit Muscle Technologies offers a defrost hose kit. Other vendors have pretty much the same kit. Not exact OEM reproduction. OEM hoses had 2 different diameter ends so drivers side of windshield would get lions share of air. The kit includes adapters to make the same single diameter hose fit all 4 places.