What I would do is take some cardboard and start drawing out some templates. Cut them out and glue them togheter, place them on the existing hood and go from there. Once you'll have a first rough copy you'll see if you want it more curved, higher or smaller etc. If you dont have a metal brake I'd do it in 3 pieces. 2 identical side walls (which will end up looking like bent triangles) and 1 piece that goes across.
Mike
Mike