well recently both me and my brother have added W concave to our boards, we both did it with balsa wood. i got 6mm and i think jesse got 4 or 5 mm. anyway, balsa wood is really good because it is so easy to shape with just sandpaper. but really you could use anything.
so yea i chucked it on my board with a tiny bit of shoe goo underneath (only source of glue within arms reach)to stop it sliding round as i put grip over it. once i had gripped the back bit i realised it was a bit too obtrusive so i cut the piece off and sanded it way down to about 3-4mm. mines only about an inch wide and jesse made his way bigger and hopefully he will post some pics.
so yea here it is.
http://i997.photobucket.com/albums/af96/doig1818/my%20boards/helmetandWc...
http://i997.photobucket.com/albums/af96/doig1818/my%20boards/helmetandWc...
oh and i did the funky grip because i didnt have enough to fully cover the last section without leaving about a half centimeter gap so i just cut up the grip into a pattern.

woo that was way too much typing.