Sounds like you can patch it. You need to cut a patch piece from same
thickness plywood, slightly bigger than the hole.
Then trace the outline of this patch over the hole and cut a matching
hole in the ceiling. This patch should fit very snugly into the hole
(make it a little snug and use sandpaper to make it perfect).
To hold it into the ceiling, you have to put some "wings" on it. Take
a couple of thin boards (like heavy duty paint sticks) and glue to the
back of the patch. The ends should extend a few inches outside of the
Now this piece will sit right in the hole, with the wings holding it in
(and possibly the insulation adding pressure). You can either leave it
like this, glue it in or attach with some screws (through the wings)