Many people who have a caravan or big hanger hooked to their car use extender mirrors.
These are mounted using rubber straps and plastic clips while they do hold a lot they eventually break.

Thats what these clips are for they are a strong geplacement for the origional clips allowing to have spares while on the road or on vacation.
I have strength tested them and they are tested practical in normal use mounting 2 mirrors strapping them thight with force they did not break.

I printed them in Black PLA from R3D and once from Jupiter Series from 123-3D
I always print them in 100% infill for strength with 6 walls and a straight infill.

My print settings:
infill 100%
Straight infill pattern
0.4 Nozzle with 0.2 layer height
Seam position aligned
Speed is up to 300mm for inner walls and 60mm for outer walls