I was having some issues with the tolerances on my printer, so I opened up the card slots a little bit. Currently printing it.
Update: The original versions of this thing here are opened up by 0.5 mm in every direction which, on my printer, makes the cards fall out. Now uploaded new versions ("_tighter") that are only loosened by 0.25 mm in every direction. Seems like I overestimated the extend of grinding I had to do on the ones from the original thing to make them fit.