I would cut the drywall so that you can frame a proper opening -- a frame that will let you firmly attach the ceiling portions, with enough overlap to firmly fasten drywall to the inside of the opening.

THEN, use drywall J-channel around the perimeter of the opening and around the perimeter of the filler piece.

When you install the filler piece, you'll have nice straight edges and a tight professional looking seam. It can all be painted in the ceiling color and not that noticeable. If there ever are any plumbing problems, the middle panel can be cut out and removed and a new one made.

The OTHER OPTION would be to find some kind of ventilation grate the same size as, or bigger than the hole. Paint everything behind the grate black and install it. Everyone will assume it's supposed to be there.