I bought an "unrepaired" Ender 3 Max off of eBay expecting to have to put some work into it. Sure enough the frame for the Z axis isn't square to the base because the guy who couldn't get it to work torqued down on the bolts so hard he deformed the aluminum. I modeled this part to help align the rails and hopefully keep them square.

At 80% infill on my printer the parts are a snug fit. However, I left a little bit of tolerance on the set of holes against the Z rail to accommodate a shim I and others have had to use to correct a width issue on some Ender 3 Max printers. This bracket works on both my shimmed and unshimmed rails.

This probably won't eliminate the need for support rods but it certainly firms up the bottom end of the rail.

Also needed:

8 M4 button head cap screws
8 M4 t-nuts

Don't let the file name throw you. I'm extending the Z rails to get a few more centimeters of build height for larger projects.

Creality

Ender 3 Pro

No

Yes

.2

80%

Flip this so the slot inserts are on the bed. Tree supports in Cura worked best for me.