The outside of my turtleback has a spar cap laid into it, with a metal hardpoint at each end. This will provide some extra rollover protection, because my head rests are from a car and probably won't do as much. The hardpoints serve two purposes. First, if the plane is upside down, pressure on the top of the canopy will make it try to spread. The hardpoints extend below the bottom of the canopy into slots just inside the longerons. This will prevent the canopy from spreading and then collapsing. Second, they'll serve as a mounting point for bolts that I can use to set up my canopy latches.