MK3/S/S+ Heat Bed Cable Strain Relief - Short 60°

Added a 60° tail to the MK4 version of the strain relief.  Made some additional cosmetic tweaks to the original files.  Model is backwards compatible with MK3/S/S+ heatbed.

Print per Prusa recommended settings.

Update 10/6/23: The …btm.stl file crashes PrusaSlicer 2.6.1.  The problem is with the arachne perimeter generator. Switch to Classic and it should slice fine.

I've reported the issue to Prusa here: https://github.com/prusa3d/PrusaSlicer/issues/8972

If others are experiencing the same issue, please update the ticket on github.