Any type of plastic you NEVER want to use a screw/bolt that has a tapered head, such has a flat head (drywall screw) thats made to be countersunk. You want to use a head that is flat on it's bottom. To flush these you use a couterbore. When you use one that is tapered, you create a sheering/wedge effect when tightening down the screw/bolt and you introduce lots of stress into the hole. Heads with flat bottoms don't introduce this stress, instead they put all pressure downward, instead of down and out. Plastics don't like high stress point areas like screw holes or 90 degree inside corners. Making inside corners with plastic you want a radius, this helps with stresses and saves cracking.