First, I cut down some plastic Suja juice bottles, which I thought had an interesting shape. Since I want water to be able to easily drain out of the planters, I glued a piece of a drinking straw to the bottom of the bottle.

For my inner shell, I used an old film container, but you could also use an empty cosmetics bottle, or whatever you have around. I glued the inner shell to the straw, and some chopsticks to both shells to keep the inner shell centered.

I made one at a larger scale, with the juice bottle as the inner shell. I can't remember where the harder plastic container came from, but it was time to clean the studio and get rid of unnecessary stuff. Plucking things from a junk drawer to make them useful one last time before recycling is one of my favorite ways to declutter (albeit slow).

I had a can of mold release kicking around so I gave them all a quick spray. This step is optional and I'm not sure it helped in the end, but typically you can substitute a vegetable oil cooking spray for mold release if you really want to experiment (though the oil may stain the concrete).