tl;dr: Since mid March we are shipping the Mounting Kits with new plastic washers. They allow for an easier an sturdier mounting of stacks.
In the past it wasn't possible to mount a stack with tightly screwed spacers. The distance between Bricks is approximately 12.3-12.4mm, while the spacers have a length of 12mm. It would have been possible for us to order spaces with the exact length, but during testing we found out that the distance between Bricks can vary slightly (+-0.1mm). We think this is the case since that the board-to-board connector are never soldered on completely the same way. During soldering they may "float on the tin" with slightly different heights.
To remedy this problem, we got custom plastic washers made for us. They have the exact height that is needed for the average distance and they can be easily squished together by 0.05mm during the mounting. This way we can compensate for the tolerances (this was not possible with the metal washers). Additionally the new washers are non-conductive.
We advice to put the new washers at the top side of the Bricks during mounting, see below.