Rail Stop - MGN9

A small bit that prevents MGN9 carriage sliding off the rail.
Adapted from: https://www.thingiverse.com/thing:2752287

- NOTE: I print with a 0.4mm nozzle so I specify 0.4mm external perimeter width in Slic3r. TO ALL OF MY MODELS TO DATE. Typically a 0.4mm nozzle prints it's external perimeters at 0.45 to 0.48mm by default, so unless you instead specify 0.4 you will probably find mated parts are too tight and bolt holes do not fit bolts. Specify 0.4mm external perimeter and you should get the results I do: perfect fit.
- Why do I do this? Because it makes the math and the modeling easier and more precise than needing to add 0.45 to 0.48...divided by two (0.225 to 0.24) to everything....0.4 divided by 2 is 0.2... and easier.