It should snap in place firmly. Some force may be required at first.
If you can't get it in or it's too loose, scale the model in the slicer accordingly (and maybe consider calibrating your printer for dimensional accuracy while you're at it…)
The author remixed this model.
The original model has a hole at the bottom, which defeats the purpose of this device that's supposed to prevent dripping.
I created a simple cap that closes the holes at the bottom, yet still allows for easy cleaning.