i had tought about putting a mount in the original deck, and another one on the new deck, to add stability... but maybe that is overwill and you are right, i just have to cut a hole on the new deck, which will provide a tight fit on the seat pole, and mount the standard plate on the original deck.... the poles are 1 1/2" right??
and yes, i plan on using wooden 2 x 1 1/2s for the bracing, and 1/2" plywood (marine, if i can find it) for the deck... i thought about using 3/4 plywood but since its a really small deck, and it will be braced on all the outisde edge, the perimeter of the cut for the trolling motor´s controller, and the perimeter of the current storage spaced, i think 1/2" would be fine.... what do you think??
Yes, all of the one's I have ever worked with have been, just make sure you have the pole in hand before you cut the hole just incase the pole is larger or smaller than planned, not all manufactures are the same.
1/2 inch plywood is fine, you can treat the surface of the wood with sealant or even coat it with primer paint to help protect it if you can not find marine plywood.