I was looking for a much simpler design that didn't result in Z height loss using MGN12 rails on the Y Axis. I also wanted to use two MGN12 rails as having just one caused my bed to rattle left and right and wasn't stable enough for my liking (YMMV). I remixed the Drill Guide for my Ender 3 Y Axis Plate (newer, non pro version).
Thanks to bobblejot for the awesome, original design. Visit bobblejots original design for the other parts for this mod :-)
Update: January 10, 2019
Added two M3 holes to each MGN12 mounting bracket for extra stability due to the extra surface area and to further prevent any left/right play when screwing everything down to the Y Axis bed plate. Also added an updated Drill Guide to account for these extra M3 holes.