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From the photos I found of the Ender 3, seems that from the Y axis has been used a 40x20 profile, placed in vertical.
I can suffest you to start from my model of the X Axis (the working surface between the tensioner and the aluminium profile is the same), and add a bracket on the right side.
I already post also the STL files, so you can take measures and everything you need from these files.