With the cleat above the uneven deck joint, the backing plate took some creativity.
I first filled in any voids with thickened epoxy.
Structural FRP was used for the backing plate. I epoxied together two 4" lengths of 3" x 1" x 1/4" U-channel FRP to create a surface 6" x 4".
A thick layer of fiberglass was added to take up the space between the joint and the bottom of the deck.
The surface was covered with thickened epoxy to take up any empty space above the backing plate, when lifted into place. A jack makes a nifty third hand.
With the bolts in place.
I used a length of the U-channel filled with thickened epoxy to raise the cleat to the level of the toe rail. The block still needs to be painted.
View from the side.