I like the idea of something that holds onto wires through the drainage holes, as long as the pots can be attached/detached readily.
In the mean time though, for trees that haven't been repotted and set up for it, I've decided on my solution as I was getting tired of moving the plants in/out of shelter.
I didn't really like the aesthetics of normal bungee cords. I picked up some black 3mm shock cord and mini clips, and I'm making up a few cords as shown below. A bit neater, easy to tie down/remove as needed, I can make them the lengths I need for different pots (they stretch a fair bit though so suit a range of sizes), and hold the pots down rock solid.
The shock cord is marine rated for UV and water resistance.
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