Documentation of build at: https://burningbecks.wordpress.com/2018/12/25/new-y-gantry-stacking-the-pulleys-again/
As Libre3D.com's auto-compiler collides with uploading derived STLs parallel to the SCAD file that generates them, you man want to look at YouMagine: https://www.youmagine.com/designs/complete-tronxy-x5s-corexy-y-gantry-set
This OpenSCAD file generates parts for the linear rail conversion of a TronXY X5S (or potentially any other CoreXY printer made from 2020 aluminium extrusion).
The pre-compiled STLs are meant for another 450mm long rail on the X axis (the .scad can generate a variant for 400mm rails) and threaded sleeves as idler axles.
If you can't find threaded sockets / sleeves with outer diameter 5mm, length 30mm and M4 inner thread you can still fall back to M5 screws - just compile it accordingly with OpenSCAD version 2015.03-2.
The "ABS_support" parameter and the ...ABS.STL are meant for tricky materials like PC-ABS (Lilypads and extra support are designed in).
Remove or exchange logos according to your gusto.
If you don't have "GT2-20T-6-5B" (GT2 pulleys with 20 teeth, 5mm bore for 6mm wide belts), change parameters to yours, but watch for alignment with motor and corner pulley mount! (Suggested read before designing yourself: https://drmrehorst.blogspot.com/2018/08/corexy-mechanism-layout-and-belt.html - I know my carriage is off! It's up next. ;-) )
I'm expecting you to have a rail alignment piece at hand - if not you can find one e.g. as "rail_too.stl" at https://www.thingiverse.com/thing:3247870/files or at https://www.thingiverse.com/thing:2804412
Besides https://libre3d.com/category/603/Accessories/listings/1136/TronXY-X5S-reinforced-motor-mount-without-raise.html you will also need one "Tronxy x5s reinforced left motor mount by xxdanmurphyxx" from https://www.thingiverse.com/thing:3200434
=> AND ONE OF THE MOUNTS NEEDS TO BE PRINTED MIRRORED! <=
Besides threaded sleeves, idlers (eight with teeth, two without), a belt that is still long enough and the usual mounting material (M4 screws and T-nuts for the 2020 extrusion) you will need M3x16 screws and nuts to mount the idler plate to the gantry plate (also M3x20 if you want to add the X-profile-brace) and for each axle an M4x16 and an M4x20 screw. (Cylinder- or pan head prefereably.)