I wanted to add cross brace frame gantries to my Ender 3 pro for stability (similar to the CR-10 frame) and to ensure that it's squared.

I purchased these 250mm gantries from AliExpress:
1PC BLACK 2040 European Standard Anodized Aluminum Profile Extrusion 100-800mm Length Linear Rail for CNC 3D Printer
https://s.click.aliexpress.com/e/6Au8jmo

I used 32x M5x8mm fasteners and t-nuts to mount the brackets onto the sides of the gantries, and 16x M5 fasteners to mount the bracket to the frame gantry ends.
These will be a TIGHT fit by design because I figured having any play in the bracket would be counterproductive to the idea of adding the crossbar gantfies in the first place.

EDIT: you want to make 2 of the braces mirrored. I forgot to add the mirrored files for the opposing side. Will add shortly.
EDIT2: Added mirrored bracket file for opposing side.