If you want to just use a piece of sheeting to make the tip shape then it would take braces or fillets. We were discussing gluing on blocks and carving/sanding them to shape. It can be done either way.
Are you wanting tips more like shown here:
http://image.rcuniverse.com/forum/up...64/Rx96142.jpg

I've also just taken a thick piece (1/4 in or more) of balsa and cut it to roughly the shape of the end rib, glued it on and sanded it to the rib profile with a minimum of rounding. That also works fine.