I think mine is the outer (aluminium I think), then marine ply, then rubber on top, with the ali slats running across for extra grip. My friend replaced her ramp, well the wood inner with marine ply and then put the existing rubber back on top.
My ramp is Brazilian Plyboard 1inch think with cocount matting on top as when it rains this matting does not get slippery and I got mine from Derby House.