Infill is 20% and organic support was used on the "small shelf support".
The "small shelf supports" do have 4 printed m3 threads each.
Since no pulling force is applied to the shelf, the printed threads should be strong enough.
My printing was done in PETG but I suppose PLA will do as well.

Use 8x m3x10 screws to connect the shelf to the supports.

I already had the box-containers in stock.