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InfluxDB
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If you have access to 3d printer and can do basic soldering you can make one yourself, so it'd be even cheaper, since it's based on jFedor's flatbox, which itself is a variation of many of his prototypes you can find on thingverse.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.