I am trouble seeing how the ply will fit to the boat. Any chance of a picture?
Assuming, dangerous, the image I have in my head is correct.
If there is no support under one end. You could glue a straight square section timber under the ply to hold it flat, in such a position that it does not foul anything else. Alternatively, if you cannot really see into the superstructure, you could glue similar pieces of timber to the inside walls of the superstructure to provide more rigidity to the walls, and then glue the ply to that.
Solutions might depend on if the ply is curling up or down at the edges.
Without a picture it is hard to advise.