All of the above are good ideas, and here's one more idea . . .

Another alternative for glassing over foam is to use uni-directional carbon fabric in the high stress regions, epoxied direct to the foam. Typically the uni-fabric is applied at the wing root. The shape of the uni-directional "patch" at the wing root would look like an ellipse or half of a football's cross section when viewed from above the wing. Uni is only 0.005" thick.

The uni-directional would be oriented with it's fibers parallel to the wing's centerline from root to tip.

Then, epoxy on a layer of 2 oz fiber glass, over the entire wing, with the weave at about a 45 degree to the wing centerline.

Resultant is a very lightweight, stiff and strong structure. Make a sample core this way and see what you think. I believe you'll be impressed.