I downloaded the Ender 3 Y-Axis mounts from another user but they were actually for the original Ender 3. The left rail fit fine but the right rail needed to be modified slightly to fit the bed and trigger the end stop.
See photos for comparison to original and how it fits onto the printer.
Much thanks to the original author.
The author remixed this model.
Added a small overhang and moved trigger to proper position to initiate the end stop on the base version of the Ender 3 (the one with the skinny middle rail).
Uses 300mm MGN12 Linear Rail Guide with a MGN12H Carriage.