Mitch's concept of taking marine plywood, maybe 8 inch strips , and laminating them to the deck and side of the hull will be way better than what you have. This would form a mini bulkhead.
What you have done so far may stabilize the deck but the sides of the hull will still be jammin'. And next you will be .
Reinforcement must be done evenly to distribute the stress or flexing.
Next thing I would do is take it a little easier on the boat. Apparently you are in some water that is beyond the builders imagined limits.