sab said:
Try this. I don't know if you were working in Fusion, but that's what I work in. I provided the Fusion file, too, so you can look at the history to see what I did. You may not have realized that you need to convert the mesh to a solid. STL files are hollow, so if you modify them, you leave holes in them. Convert to a solid, first, then modify and save as an STL.

I also work in inches, not mm. When I opened your file, the divider was 31" or so tall, so I scaled by dividing by 25.4. When you open it, you may need to scale it back up to mm using a scale factor of 25.4. If you have trouble with this file, PM your mobile number and I'll call you to walk you through it.

Very close.

Red version is scaled using the Anykubic slicer which is very close to fitting but the taper is misaligned.

Black is scaled using fusion but fails (twice) when printing the second end.

FWIW the unmodified version of model we’re modifying printed multiple times and fits great.

MLS SAB is scaled in fusion
Other is the slicer

This is what I’m trying to modify.
https://www.thingiverse.com/thing:6460823

Custom Scaled Antenna Base Delete Plug

Often appears with