Cube and wall modifier

In cura settings turn off 'keep models apart' and 'drop models to build plate'
Place the 30mm cube at 0,0,0
Place the wallModifier object at X=15-WallThickness, Y=-15+WallThickness, Z= -5+BottomThickness
Set default settings to zero wall thickness and zero top/bottom thickness.
Set wallModifier.stl per-modal settings to:
Infill mesh only
Wall Thickness = WallThickness
Top/Bottom Thickness to BottomThickness.

[In the example pictures, WallThickness is 0.8mm and BottomThickness=0.8mm.]