I too am building the Unionville 8ft Beaver, and my approach will be to toss the plastic tips, and build them out of balsa. Then too, I am sheeting the entire wing with 1/16th balsa, so it will not be too much of a problem for me.

One approach may be to insert 1/8" balsa or light ply inside the plastic tip, so that it is flush with the inside edges all the way around. Then glue to the end rib, and fill and sand as required. By having wood inside the tip, you can sand away the plastic and then blend into the end of the wing. If it will require a lot of plastic to be removed, increase the thickness of the insert as needed.