I've ditched the plastic moulded wing tips in the past, and just used balsa blocks, carved and sanded to shape (looks petter when covered too!!)

I've used trinagular blocks, as well as solid 2"x2" blocks. Just cut them roufly to length, then glue them in place, and carve, sand, carve sand untill you have what you want. Some people tack them in place, then hollow them out to reduce weight