it wouldn't be too hard to do this for a couple of feet along the duct, then skin with 3/4 hardwood plywood. Attach the projector mount to the box.
This is what I will attempt to do.to save head room I would ladder down on each side of the duct then span under the duct with only 3/4 furniture grade plywood. Attach the mount to the plywood.
This is all correct, but when doing temporary positioning measurements for an eventual ceiling mount, I prefer to have the orientation set for the final installation configuration, even though the picture would be upside down during the testing. Theoretically, changing the orientation will just flip the picture and it will be in the same spot. In practice, it may shift up or down. Positioning is critical for short throw projectors (especially ultra short throw projectors), so I'd rather play it safe and eliminate as many variables as possible. ;-)everything Dave said but you should be able to go into the setup menu and pick table versus ceiling mounting orientation. New out of the box it is usually assuming a table top orientation,