This is to mount existing storage boxes from Allit (Toom) for example: https://toom.de/p/profiplus-stapelsichtbox-box-3-blau-235-x-15-x-125-cm/1458842
Onshape 3d model: https://cad.onshape.com/documents/15799bd682ae7d5242aa692b/w/f9bb87250bd3dc5bb55a20d6/e/f126081485ef8556153701c0
Note: Print direction is important as always! Print them as in the STL.
The author marked this model as their own original creation.