Import the STL into SketchUp.
Prior to import, look under Options button.
There, set the units to Meters to avoid SU’s issues in creating tiny new geometry.
The STL comes into SU as a Component.
Explode the Component.
Make your modifications.
Make it a Component again and confirm via Entity Info that the model is a Solid Component.
Solid Inspector can help you find flaws preventing it from being a Solid.
https://extensions.sketchup.com/en/content/solid-inspector
Scale the model down to real world size and export the STL file.
I imported your STL into SketchUp in the manner described above.
The model has many holes.
It’s not printable unless it’s a Solid; an airtight closed volume.
