ive made a few blades, your idea to make them of glassed ply should work. i usually use layers of marine ply epoxied together to get the right thickness. i suggest trying to find used ones on the net first, try classifieds at catsail.com or sailingproshop.com. fabricating your own may cost as much as buying used ones unless you happen to have some extra epoxy and suitable ply on hand. i made a decent centerboard for an albacore daysailor from 2 layers of 1/2" 5 ply wood glassed with epoxy that has lasted over 3 years so far- gluing, carving and glassing several foil sectioned boards could be a lot of work. good luck, hope you get your boat going soon!