Don't use Balsa fins! Nike's have alumium plate folded fines with a center or two internal spines. I've built up to BT-80 size nike boosters using the same method in cardstock, cardboard or event 3/64" Plywood. score the card or ply on the inside and fold. CA/epoxy the spine(s), then apply the second side.
If you must use balsa or basswood the best way is to tape the center line and sand with a 12 or longer sanding block in ONE direction only tip to toe or toe to tip until you have removed the amount of material necessary.
Nike fins also taper from tip to root, with is best sanded first then sand the center span fold.
Here's a 2 pic photo of how the cardstock fins are formed, they are more then strong enough for LMR cluster power model flying.
Hope this helps