I would test the PVC cement on the bottle cap to see if it has any advers reactions. There is no telling what plastic is being used to make the bottle caps. I would also be concerned with water leaking if it is a grooved cap.
Another option that would probably work is if you sand the inside of the PVC cap with some rough grit to get some nice grooving (parrallel to the edge of the pvc cap to have it serve as gripping) and then you can use a small amount of silicone. After it is allowed to fully cure there shouldnt be any funing to worry about. Just try to get some food grade silicone, and no kitchen/bathroom grade. That tends to have built in fungicides, that would not be very good for you or the plants.
That is just my thought on it. I haven't actually made those yet though I might try one to play around with later.
I hope my input was helpful, and I hope that someone who has made one of these either confirms or denies my input as useful. I am just trying to give you some where to start looking and thinking.
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