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InfluxDB
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Take a look at https://scottokeebs.com, he has a lot of open source designs that involve hand-soldering so no PCB required. Just 3d print a case top and bottom, add switches, diodes, wire, ucontroller, cheap blank keycaps, and some soldering work. With bulk purchases it might be below the €30 threshold (filament might push you over budget?)
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.