...if I'm reading the drawings correctly (mcmaster has CADs) is that the underside of the head is slightly funnel shaped. Like a more gradual version of a wood screw head. Much of the "funnel" would be in the hole, but I wonder if the rest of it would cause the outside edge of the head to stand proud of the fiberglass instead of sitting flush. I also can't see whether the top of the head is "nice" or not. I vaguely remember them being very utilitarian looking vs. the finished top of something like a truss head. Still good to consider as an option though, so thanks for mentioning them.