This is a brace for the Ender 5. The printer is constructed with 40MM x 20MM extruded profile where the longer dimension stiffens the X axis which has the least moving mass (just hot end and cooling garbage). The Y axis relies upon the 20MM profile to resist the movement imparted by the much heavier Y axis gantry. I suspect that this is somewhat on purpose and it lowers the frequency of vibration imparted into the structure (and print). I am planning on replacing my rubber wheels with linear rails and lightening the gantry significantly so I would like to eliminate the play in the system.

I printed the part with Amazon PETG, .2 layer height with 6 walls/top/bottom and 20% infill. The part is super solid. If you have your filament dialed in better than I do you will probably have a cleaner and less-booger print than I do.

You will need two lengths of 3/8 threaded rod. I cut my rods to 19.5" and I found I was able to get the piece in and out of the printer and tighten all the bolts easily. You will need qty 8 nuts and washers as well as qty 16 5mm bolts and corresponding T-nuts as well. Put the nuts on such that the braces can slide around and you have enough play to put the T-nuts in. Then square the sides and check repeatedly while tightening.

When installing, note that one side has a "V" inset in the groove. This side is for the vertical face. There is one part for bot sides but recognize that the printer is a cubic rectangle so the angle of the threaded rod is not 45 degrees so you need to orient the brace properly. In a standard configuration the upper brace will probably rub the Y axis belt. I don't think this is a huge deal. A little sandpaper or you can chop off that part if you like. I am planning on relocating the belts outboard on my printer so I can lock in the upper frame to the braces.

When installing you should use a machinist square or measure the sides corner to corner to insure they are perfectly square before tightening the braces down. They will completely eliminate any flexibility in the Y plane.

Edit: Thanks to comment below changed 5/8 rod to 3/8 rod.