Nubs do have steep overhangs, which do not print very well with slic3r, even though commercial slicers can do the job just fine.

Problem can be solved with use of 'modifier' feature of slic3r.

Just load supplied .stl file as modifier to sub of size 6, and then it allows you to modify extrusion width for perimeters to larger one, and same for extrusion width for external perimeters. you can also increase infill ratio, increase perimeter count and slow down perimeter print speeds .
All those modifications affect just the problematic region.

Then slice and export as usual, and enjoy better print.

For 0.4 nozzle, i do use 0.24 layer height, 0.55 internal and 0.37 external perimeter extrusion width.

for modifier area i do use 0.8 extrusion width for both internal and external perimeters. i also increase perimeter count to 3 and slow down to 10mm/s