The slot for accepting the lug on the back of the dial indicator gauge printed out under spec @ 5.61mm instead of 6.82mm that's the max width on the Harbor Freight gauge I got. Going to do a redesign and give it another go.
I suspect that Harbor Freight varies product details over time.
In any case, I posted the source code (OpenSCAD) as well as the STL file, so you can change the measurements and re-print it pretty easily. You can download OpenSCAD from openscad.org.