I don't recall it being especially difficult to do the wingtip sheeting. I do remember cutting a piece slightly larger than required and then slowly/carefully trimming the (not quite) right angle edges until they fit perfectly. Before glueing to the wing, I trimming the excess sheeting at the wingtip to about a 1/4 inch to minimize difficulty in bending. Also be sure to sand the plywood wingtip piece (sort of to a knife blade shape) so that there is good amount of bonding surface for the glue. Once the glue is cured, trim and sand.

Good luck. Thanks for your diary. It's been a pleasure to follow your work.