Finally! Thanks to the imminent Hackaday Contest deadline, I don't care anymore about making it perfect first. 

The design files, including some terrible code that makes the thing work, have been published on GitLab. 

https://gitlab.com/v3611/businesscard-battery-tester

I'll probably be using Hackaday.io for the more social side and irregularly post major releases here (like, whenever I'll order more cards I guess). Still figuring things out. I've been looking forward to joining the contest and am so glad I didn't miss the deadline.