I would say fill the holes with liquid insulating foam. You just spray it right in the holes. Not sure if I can link to other websites, but look up "GREAT STUFF Insulating foam". Its 4 bucks a bottle. You can spray it down into the holes and I assure you it will most likely seal it. Spray enough that it is coming out the ends. Then trim off the ends and make sure you have a little room left for some silicone. To ensure a seal, you could put some silicone on top of the foam.... It is foam however, so you would need some heavy weights. It is very lightweight so Im not sure how well it would float. I would say concrete in a bleach bottle would do the trick. One on each side. If you end up using concrete, make sure it's out of the way of the poor kids feet when they're getting in and out, don't want them kicking that.