Are these machine screws? I would avoid the use of machine screws for attaching plastic panels as the torque required to retain the screw is too high and will damage the plastic. A friction fit thread similar to a wood screw would be a better choice. If the panel requires frequent removal, clip nuts on the mating part will help maintain friction fit. Otherwise, bushings should be installed in the panel for support to be used with machine screws. Well nuts work well, too.