so i made just the model you were describing, a 13 inch span hang glider plan-form and it works real nice
its cut from 2mm depron sheet, total weight before cutting the skeleton is 1.9 grams, after it was 0.3 so that removed 1.6 grams of weight keeping just enough foam to hold its shape. i rounded all the inside corners to help prevent the foam from tearing (just as the skeleton planes do for indoor rc models)
the covering is a very very thin plastic bag which with the spray adhesive added 0.1 grams so its AUW is 0.4 grams.
after fiddling with the outer ribs to get the washout right, it floats across the room nicely, now i need to build a pilots frame and a pilot