Hold on Paul. One side is sufficient for constant chord surfaces but if it's tapered in chord but constant thickness then it needs to be done from both sides. For that situation you need to reference to a center line on the rear edge. The wire or rod jig idea will still work but you need a thicker wire for the first cut and then the thinner one for the second. The flex in the balsa will let the surface twist under the pressure of the sanding block.

If you try to do a single cut on a tapered surface the CENTER LINE angle will change from the root to the tip.