Here is my custom fabricated mount to put the BMG in as direct drive on the Ender3. No permanent modifications to the Ender3 are required (just extend the wires out to the carriage). Parts were cut from 2mm aluminum sheet metal on my R7 CNC. Then I bent the parts on a vice mounted press brake. Finally I powder coated them with a Harbor Freight powder coat gun, and baked it in a toaster oven.
I have my files provided in the native Solidworks, as well as Fusion360, X_T, STEP, 3D-PDF, and 2D drawings in PDF and DWG. The 2D FIles can be printed 1:1 so you can cutout the parts by hand if you don't have a CNC available to cut the metal parts.
With this modification you can print with a lot of retractions easy as pie, and flexible filament is no problem (see youtube video below of printing Sainsmart TPU at 35mm/s).
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