Extending an air duct?

I am installing a suspended ceiling in a room where the heating/cooling register (approx. 8" x 11" overall) is currently at joist level. The new ceiling is 4" below the joists. How do I go about extending the duct to mount the register on the underside of the ceiling panel? Or do I simply angle the horizontal section down so the end finishes up at the right level?
BTW, this the room I mentioned in a previous message that has a heating/cooling register but no return. (Our bathrooms are the same, I now realize.)
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MB

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level?
If you can angle the duct down at a low angle it should be fine. Usually with a suspended ceiling the register is mounted to the ceiling, with a short run of flex duct connecting it to the rigid duct. As for the returns, it is pretty typical not to have returns in every room. Often the returns are in a common area. Bathrooms normally do not get returns. Greg
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