The riggers look nice. What you might want to consider, is to make 6 circular aluminum plates with the mounting holes drilled in the proper locations and then remove the nuts on one mount at a time and replace the washers with the circular plates for backings instead of just the washers. This will effectively increase the amount of surface area taking the strain of the riggers.
Anchoring the backing plate to the deck won't give you any strength advantage in that setup. Only diagonal bracing would be to an advantage, but I don't think you need to go to that extent... Just put in plates, as Harv suggested, to keep the washers from sucking into the ply & you should be fine.
Roger that! When I was tightning down the bolts I was not paying attention and pulled one a 1/4" into the ply! That is when I knew! I should have used bigger washers! Anyhow, they seem pretty durn sturdy and I will put some circular plates on the bolts and get to fishing!