Installing a ceiling register boot

I have a small room on our top floor (the only room on the top floor) in my townhouse that we would like to close off and make a bedroom. If we do close off the room, the only register would be located outside the room. So, I have made a duct branch off the main supply on the top floor. This main supply only serves the top floor registers. So, I went into the attic, split the main duct and put in a TEE connection. I have sealed the connections with special duct tape I got from the hardware store. The hard part seems to be installing the register boot in the ceiling. I have a 12'' X 4'' register cover and the same size hole in my ceiling. I cannot seem to find a register boot that covers this nicely at the local hardware store. So, any ideas on how to properly install the register boot? Also, if I do find a register boot to cover this, how should I seal it? Is this stuff available at local hardware stores?
Sorry if my terminlogy is off, I am just learning this stuff.
I have been reading many posts in the group and have found this group to be helpful, but have not found an answer to these particular questions.
Any ideas?
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