I knocked the wingtip off my RTF trainer when I treed it last spring. Here are some pics of the repair.
Where the wingtip used to be.
I got a balsa block from my LHS. I cut two pieces to length and glued them together.
I placed the glued block next to the intact one on the other wing, always bearing in mind that I wanted a mirror image, not an exact copy. I traced a rough curve and cut it out.
Next step was to hold the block in place on the broken wing to trace the shape onto it. Then I cut that out, leaving a VERY rough wingtip.
Next, I glued the wingtip onto the wing.
Finally, I used a block plane to shape the tip. I went slowly and compared it often with the other wingtip.
The end result: