On Nov 14, 11:06 am, Renata wrote:
Plan to cut two new registers into the main trunk line running thru
the basement to add heat to the room. Current registers in that room
are "off-line". Trunk runs in the ceiling, behind plaster. I plan to
cut the hole, fabricate a short duct, put the register in the ceiling,
right off the trunk. Seems rather simple and straightforward.

(just in case) What am I forgetting?

Thanx
Renata

Seems rather simple and straightforward

A former boss used to say "Nothing is ever what it appears to be."

Depending on the layout, you may end up getting too little or too much
heat in that room based on air flow. This could impact the rest of the
house. You might need dampers to help balance the system.

It might be worth paying for an opinion from a pro.