I used an SCAD parametrized model to create my Gridfinity boxes, using the default parameters.
It turned out that a height of 42mm without Z alignment (trhough e.g. Zack's method) does not lead to the correct height and as such I run into the problem that in my closed Gridfinity box washers and other things started to mix.
In order to prevent re-printing 25 boxes, I created this spacer which allows to be put into the Gridfinity box to compensate for the missing height.
Not sure how useful this is for others - but if you were running into the same problem: Here is the solution.
The author marked this model as their own original creation.