This bracket shifts the spool toward the front by 4cm so that it is more centered on the top rail. I was uncomfortable having both the power supply and especially the filament spool pulling the verticals rearward, this causes unnecessary stress where the vertical extrusions meet the horizontal base. This mod helps provide better balance for the frame.
Getting the spool off the top rail entirely is another popular option which also improves the filament path, but due to space limitations that was not an ideal solution for me.
You will need (2) M5 12mm bolts and (2) M5 nuts.
The nuts slide into the side and may be a tight fit, but they seat well and align with the round holes.
Print it on-end, depending on how well your printer does with bridging you may not need support. If you do need support, use as little as you can as it is difficult to remove from the round holes and rectangular channels.
I used the rail connector design from MWizard64's Ender 3 spool mount as he already dealt with re-working the geometry so it slides more easily. Many designs that slide into the rails are too tight for my liking. All other aspects of this design are new.
Print time for me was around 2 hours, YMMV.
Use as little support as you can as they can be challenging to dig out.