I use a drill and 3/8 inch bit (for balls at least), pick off any plastic curls, then hit the hole from each side with one of those wind-resistant lighters. The ones with a flame like a small acetylene torch. Melts any sharp or jagged edges, and since the flame is directed into a point rather than lapping all over like a normal lighter I don't get nasty black marks. Learned that after one ugly tub. I've only ever used this method on Sterilite tubs, so I can't say how it will work on others. I've also never had any problem with cracking, and I press pretty hard, enough to bow the plastic from the pressure. Again this is with Sterilite tubs which are made of a pretty soft plastic.