This assembly acts as a sliding base for anything you want to mount to it. Keep in mind that bolt locations are already determined. If you want to attach something with different dimensions you'll probably need to edit the files.
I originally designed it as a motor mount, and the sliding mechanism allowed me to tension the belt attached to the motor.

Keep in mind, to complete the assembly you will also need 12 M4 Bolts and 12 M4 Nuts.
The bolt heads should be smaller than 3 mm in height and 8 mm in diameter. Bolt length is dependent on what you attach to this sliding base.
The Nut width should be smaller than 3.2 mm.

Don't forget to print 2 of the bar parts.

Creality

Ender 3

No

No

0.12

40 to 100%

Higher resolution due to tolerances. If needed add supports only to bolt holes.

After printing, you may need to slide the parts a couple of times to make them slide smoothly.