One alternative is to use wing skins of pre-cured fiberglass sheet. The skins are produced by laying up fiberglass on a flat sheet of glass, resulting in a smooth finish surface on one side. The skins can then be applied as you would the plastic sheet material you described. May not save any weight, but they're easy to produce. With proper orientation of the fabric, you can produce a very strong wing. This could serve as a stop-gap measure, until you produce some female molds for the wing skins that would allow bonding the cores to the skins inside the mold.