Depending on your downrigger choice and how you mount things, backing plates may be overkill.
The gunnels are incredibly thick, and at least with Scotty bases, the load is already spread out over a very wide area just from the base. What good does it do to add another 1/4-1/2" of material underneath?
On both my 19 and 22, I just drilled the four Scotty base holes, and used two large stainless fender washers per bolt underneath. It's insanely beefy...the downrigger extensions would bend, or the cable would snap long before the bolts/washers would pull through.
Personally...I don't see the need for backing plates anywhere on the fore deck or gunnels on a C-Dory. Neither the factory nor Les at E.Q. even use them for windlass installations, which would seem to generate far more load than downriggers. To my knowledge, the factory has never used them on any of the cleats or the bow rails, and I've never heard of a case where this has been problematic.
Now...either the cabin top, or the motor well are a different story.
The above being said...by all means, if backing plates make you feel warm and fuzzy, have at 'em...
