DIY Brine Shrimp Hatcher/Enricher
 

I have tried this. While it does work i find that it is not very sturdy. I bought for 10 bucks a plastic stand that a bottle neck screws into and has air into the bottom.

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I've been using them now for well over a decade with no problem.
It is important to cut the bottom cup so that the bottle cap of the upper portion touches the bottom of the cup and the sides of the bottle contact the lip of the bottom cup all around.
You can always put two bottles together with a plastic clothes pin for more stability.