I had designed an X axis carriage for the Ender 3 Pro: https://than.gs/m/1492151

As an addition, or rather option to make the X carriage work I have designed appropriate brackets that mount on the back and top of the X axis aluminum profile. These haven't been tested yet but only measured against the original frame and hole slots of the linear rail. Please let me know if there are issues!

Mounting instructions: Print 3-4 brackets. Put M3 (3mm tall) threaded inserts into the recessed side of the bracket. These will later hold the linear rail. Using 2-3 M3 screws per bracket, fasten the linear rail against the brackets. Make sure to evenly spacing them and that they fit on the backside of the X axis profile.

Once in place, use M3 screws and T nuts to attach it all to the X axis profile.

Print in a rigid filament (e.g. ABS CF) 5 walls, 100% infill Use supports for looks only. Will print without. Do use a brim