Great tool, solves a lot of headaches quickly with identifying faulty cables.
Couple of suggestions (I bought the kit minus the 3D printed case as I printed my own):
1) Soldering the supplied USB cable to the bottom of the USB B power input is very fiddly due to the closeness of the 4 pins making shorts a likely posibility. Perhaps on a later revision, wider spaced solder pads connected to these 4 pins could be added in the free space on the PCB to make shorting less likely?
2) With the 3D printed case: an opening in the base for the barrel and USB power might be beneficial if the user doesn't want to use the supplied solderable USB cable (see pins issue above)
3) For another project, have you considered an audio/visual cable tester? HDMI, 3.5mm jacks, Component/Composite, TOSLINK etc? I know some HDMI cables have circuity inside that may make them incompatible, but basic HDMI should work?