I dont think i would totally dismiss the foam thing some of the super light but still strong gliders i used to fly were based around a glass/carbon d box and trailing edge with very light carbon caped ribs. with then the main area of the wing covered in mylar. You could very easily do something similar by hot wiring cutting you wind in blue insulation foam then vacuum bagging it with glass or carbon and then machining away pockets to leave a rib type structure. That way you wouldn't have the expense of creating moulds and could try different designs easily and while carbon sounds great it is also 5 x the price of glass as i have found in the past you can often get away with using very small amounts to get the required effects in weight and strength and substitute the rest for standard glass keeping the costs down.

Do a search for highlight rc glder and you will see the design i mean which will give you by far the strongest and lightest structure suitable for what you want.