You see, many years ago (pre-blog), I had my upstairs bathroom redone. As part of that work, they needed to access the pipes & wires in the downstairs bathroom ceiling, which was done with those foam tiles that get stapled up. They cut a hole so they could take one down (with my permission).
Of course, you can't really fix that well, and the ceiling has been less than ideal ever since.
The main room downstairs has a similar problem, but it's even more complicated. (I want to move a bathroom wall to create a bigger shower, and move the door, and ....) Since I was just re-doing the bathroom ceiling, I needed something that would work on a small scale, but that could be removed if necessary.
I pulled down the tiles and bought some 1x4s. I painted the boards white and started screwing them in place.
Half of the boards went length-wise, half across the width.
When I was done, I caulked the gaps and painted it again.