Plastic floats are not really considered an emerging technology. Are you sure that those did not come with ply or hardwood reinfocements INSIDE? The Falcon Trading/ModelFly/Hobby People floats have a wood spine inside the raised flat area on top of the float for just that reason.

Hold 'em up to a bright light. Stick a wire down the screw hole. DO whatever it takes to be sure that there is not already an inside reinforcement part. If not, PFM/ShoeGoo makes a pretty good adhesive for sticking ply to a slick surface. Put a glob on, slap on the ply plate, and tape it securely in place. Check it tomorrow and it will be >>Attached<<<.