Plastic films do not stick very well to epoxy or polyester resin. A coat of Balsarite over the resin really helps the fim to stick. Even with a glass covering if the sheet underneath is fragile it will still fracture under pressure. Cut some slots in the sheeting and fit some sub spars between the ribs, sand flush and then glass.

Be very careful with expanding foam. It is more likely that as the foam expands it will blow the wing apart. The area where the foam is to be placed has to be clamped to prevent this. I would advise against it.

Either way however the amount of weight that will be added will be negligible.

Ed S