The underside of decks..or the inside of fiberglass hulls are Always Rough/Uneven surfaces. Metal..aluminum backing plates would be IDEAL..but only under Ideal conditions..a smooth/even surface underneath. For this reason WOOD is preferred because it is able to conform to the rough underdeck surface...much like a mattress conforms to your body.
A backing plate is usually ONE Piece of wood or metal with ALL the bolt holes drilled through it. An alternative in a difficult underdeck situation ..would be large stainless steel "Fender Washers" on each bolt.
NB