They are 10 mm wide and consist of a stack of two dividers scraps, hence 30 mm high and 2 × 6 mm wide, stuck together with double-sided adhesive 1. They are also stuck in the angles formed by sides and dividers. Their role is to make stop at the buffers level to avoid deforming the couplings.
Tip: apply the double-sided adhesive on the extruded polystyrene strips and glue two strips together before slicing them into 10 mm small pieces!
Only one shim may be put like here, or one on each side for more safety, and also for a better vertical support of the dividers.
Mass of the finished box: about 250 g