I purchased–and used–two professionally prepared enhancements for the bottom of my large planters. My name for them: “air bottoms.” I purchased “on sale, half price” but resented the $4. price. So I started making my own from plastic bowls and trays. I’ve found Goodwill a good place to find “candidates” for this project. ~~  I save in one area to offset the expense of mealworms, black oil sunflower seeds, wild bird seed, squirrel food, etc.!!  ~~  FYI: Since I learned the merits of these inserts, I no longer use river rock in the bottom of the planters.  (I gave away about forty pounds of river rock.)