If you are planning on using the standard violin tailpiece and a bridge like a violin, or a uke style bridge, you won't need the sound post for either. The sound post in a violin is meant to activate both top and back plates together and since a violin is a bowed instrument, that makes perfect sense. The string downward pressure on a violin is also greater than a uke. This uke you are putting together is going to be a plucked instrument and therefore you don't want the back to be envolved accept as a reflective plate. No sound post is needed and will in fact hinder the sound if you install one.