We had 19 bays total to fit out with fascia trim for both the living and dining room ceilings. We were able to pre-assemble (and lift into place) four-sided boxes in 11 of the bays; in the remaining 8 bays, we installed the trim in place.
As we completed the boxes and allowed the glue to dry, we installed them in their respective bays. In some cases, to press the lip tightly against the bottom grid, we had to use floor-to-ceiling springboards.
Strings stretched in each direction were installed to help line up the boxes before permanently attaching them.
We shot finish nails through the side panels into the framing. In some cases, we added trim-head screws, later concealing the heads with 1/4-inch-diameter bungs. Generally, this strategy worked pretty well, with only a few seams refusing to close tightly; these we filled with paper-thin rips of cherry, which would be invisible in the finish.
To close some of the more troublesome seams, we applied yellow glue.
At outside corners, the two-way projection of the false ceiling’s perimeter prevented the insertion of a preassembled frame (a minimum of three sides clear was needed), so the trim build-out had to be installed in place. We were able to preassemble the elements of one side and one end of the trim, but the rest of the box had to be put together in place. We glued down the bullnose pieces individually all the way around in those eight corners, with no visible loss of quality; it just took longer.