If you have yet to build a tiller I think that I would go for an S shaped tiller. Keep it horizontal at the stock to preserve the depth of the stock, then an angled section to bring it up over the transom bar, and then back to horizontal so the tip does not go too high and drag the extentions.
It is worth checking to see if the tiller hits the side deck before getting the carbon out, as this may restrict your steering angles. You could do the angles as Will suggests by cutting a tube, however using foam would let you use curves which might look better.
