I had to make a bridge to fix a dished/bowing problem. Kind of extreme but I couldn't get a clear path across the playfield to fasten a metal support.
In your case if your cabinet has full length side rail supports that the playfield rests on (cabinet support rails, not playfield rails) then you can add screws along the playfield side rails when it is installed that will attach to the cabinet support rail underneath. This may flatten it out and over time may settle in better.

Alan

bow fix (resized).jpg