Uses a pair of mgn9 350mm rails with H (long) carriages.

This will only work with the BNBSX or Prusa R7 extruder. There are STP files if anyone wants to do further mods.

Running two MGN9s may be a bit of a challenge, as you have to be pretty accurate with tolerances, but does have some benefits. The carriage is now super stiff. Much less flex back and forth than the standard lm8u bearing carriage. The lower rail also allows you to mount the cable support directly to the bearing, so it can't twist the lower part of the carriage.

Update 31/07/22 - Added a version for the Prusa R7 extruder. Not sure about BOM, but it's mostly a few M3 screws. Use low profile (button head) ones of 5mm length for the lower rail/carriage connection.