I noticed some flexing of the transom as I would board the boat using the transom mounted ladder. I replaced the original plywood backing plates with 3/16" thick stainless and used acorn nuts to help prevent snagging of items placed in the lazarette locker. I also added some fiberglass to stiffen the transom where the ladder is mounted. I laid on a couple of layers of biaxial cloth, then added 2 foam forms (cut from 1" pink foam insulation with a razor knife) over which I added 4 more layers to create some vertical stringers to further stiffen the transom. I also added a layer of mat in between the 1st and second layer of biaxial to thicken the lay up a bit in the areas of the backing plates. The layers of biaxial taper away at the edges so there is no distinct shift to the thicker sections.

Backing Plate Measurement Drawings
Note that the bolt spacing is slighly different for the ladder mount and the transom step.
boarding ladder backing plate
transom step backing plate

Hardware Sources
McMaster Carr . . . All types of harware and materials