The shim increases the tension of the bed springs. I found them too loose on my build, and played a bit with different shim thicknesses. 3mm is a nice fit for the yellow standard tension springs.
Print these in ABS or better for obvious reeasons :) I used Polycarbonate, as it resists even more heat.
I made a triangle version as it matches the Kirigami. The round version is functionally the same. Pick what you like :)https://cad.onshape.com/documents/b3dab41fc3c48bbcbf576088/w/5fd5dcd926104f53a0641cd8/e/68c04d108a48969cead9721b?renderMode=0&uiState=6352cccba10e15610f329d47
The author marked this model as their own original creation.