Because it fits into the table, it is easier to attach.
If you can find the right size planter to fit inside your table, this apparatus is as simple as the pail from an earlier post.
Planters always have holes because it is important for plants to drain. I could use those, but they tend to be too big so the tray won't hold any water. For that reason, I keep the holes plugged that are part of the planter and I drill two to four smaller holes on the edge of the bottom. That lets the water out more slowly and provides another avenue of exploration. The child below is filling his plastic syrup jug from the hole at the bottom of the planter. Notice the bottom of the little jug is in the water. That means that at first, the jug is buoyant and the child has to push down. As the jug fills, the child has to reverse the action because the water filling the jug gets heavier, so he has to hold it up.