I recently bought the new Bondtech LGX (Prusa Flexible edition with plastic parts) directly after the launch. Since I have rebuild my MK3S to a bear build with MGN12 rails, I needed to adapt it to fit my printer. I asked Bondtech for 3D models and they kindly supplied me with what i needed (and actually asked me to share my design. I love it when a company supports the community!).
My general idea was to use as much of their parts as possible but make it work with bear parts and MGN rails. In my solution you remove the bondtech supplied front- and back X-carrier and replace it with my 3 parts. For the adapter piece I use 4 threaded inserts.
The yellow pieces shows an older version but you will get an idea of how it works.
The design uses the original Prusa strain relief for the extruder cables.
I use the LGX firmware from Bondtechs homepage.
I printed with supports.
I have probably forgotten something, give me a shout then. Happy LGX printing on your MGN12 updated MK3S!