Vanessa or any of its simpler variations (sling & plumb bob) will do the trick. Just hang it, adjust it to straight & level, adjust the plumb bob so it comes to rest just above the surface of the model. Mark the desired CG. Plumb bob will point to actual balance point. Just move things around on the hanging model, rest them on top or tape them or hang them in position, and keep doing that until the plumb bob points to your desired CG point. Does lengthwise and lateral CG simultaneously. It is important to have everything (as in both wings) in position to get accurate results.