I'm going to give these a try:
https://www.amazon.com/dp/B007XIWW4Y/
They can be stacked. Maybe make a cross-piece for the bottom of the pot. And/or, in a plastic pot, I think you could drill some holes around the top of the pot and arrange some anchors--rope or wire.
Will have to add some ballast to the bottom of the pot, I mix some gravel in with the soil.
