I saw a picture of a brace for 20x20 al extrusions, but I couldn't be bothered to go out in the rain and buy screws, so I made a brace that fits into the grooves of the frame. It's a tight fit, but it works without much effort.

This works for a 20x20 extrusion profile, and I have them mounted on my CR10S-PRO.

Files are availble on TinkerCAD by request.

UPDATE
I made a new version with double legs, just to make it even more rigid. There is one version for the left side and one for the right.

Creality CR10S-Pro

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0.2

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