I suggest getting a couple of feet of 6061 aluminum C channel. Per ASTM B 308 with the rounded inside corners. I used 3 x 1.75". I put 2 feet down in the bilge. Use that as your "washer". It will transfer the load across to the box stringers. If you can handle the look, 3 feet of the same up in the splashwell will help spread the load there, too. With a little handiwork with a file and drill, you can use it as a cable trough for the outboard wiring harness and transom mounted depth sounder wiring harness.

To me, it makes more sense than trying to crush plywood under a fender washer that is distorting under the bolt load to become dome shaped.

I used this: http://www.mcmaster.com/#1630T321

I have used 4" x 3/8" aluminum anodized flat bar before as a backing plate. It will look a bit better but not as strong as the channel would be. You can find this any where they builds t-tops. It is used for mounting t-top mount outrigger and for other support on boat towers ect. It would help spread the load much better than a fender washer or such. :)