These (or something similar) are the weights I use
Pool Supply Unlimited has some of the best prices when shopping for Blue Wave Easy Step Weight System | Pair | NE1129
www.poolsupplyunlimited.com
My stairs are at least 5, if not more, years old. I cannot recall what manufacture makes them - I think I got them from INYO pools, but that could be wrong too. I ordered the weighs at the same time.
The weights come with brackets that attach to the sides of the stairs. The weights then clip into the brackets by sliding down into them. I can remove the weights by grabbing the built in handles and lifting straight up.
My stairs do not have a shelf under them, and once they are in the pool, there is no way to get under them. It is a safety thing, so nobody can get trapped in there. Water flows through the "bars" just fine, but not people.
Also, my weights are about 30 lbs each when full. 100 lbs of weight is a lot of weight. When the stairs themselves fill with water, they really do not have much floatation to them.
Also, I say no to sand, and use pea gravel (3/8") instead. If you get a crack in your weight container (whatever you end up using) it is a lot easier to clean up pea gravel, if it even makes it out of the crack in the first place, than sand.