I took Silver3d's heatset insert mod for the muzi.works H2T case back, and simply added a belt clip.
The clips are these very inexpensive ones from Aliexpress. They come with no hardware, so in addition to the clips, you'll also need:
Button head screws are required here since they taper and allow the belt clip to slide on properly. The recessed nuts are a pretty tight fit, but shouldn't be a problem if your printer is decently calibrated.
There's a recess made on the bottom that allows the clip to sit flush with the back of the case. In order to print this, you'll likely want to use supports to ensure it's as flat as possible. It doesn't really add much additional print time, but does make things easier when assembling since there's minimal sagging to the bridged layer.
The author remixed this model.
Extended case back several mm to allow recessed M3 nuts, created recess for linked belt clips to sit flush, and added holes to mount clip.