Rope and triangle bracing may help I s'pose, but the 3D printed corner parts are quite strong enough. I did print them in PolyMax and that is pretty tough.

And  making it with braces complicates it a bit. Feel free to mod it to your heart's content :)

in my opinion construction need rope.

every construction is better constructed from triangle not rectangle.

meybe add a hole for rope and add diagonal of a square of thread/rope. the construction will become more rigid