We present a new method to study the evolution of instability-generated structure in the outer winds of hot stars. We coin this method a pseudo-planar moving periodic box technique. It makes use of the fact that the outer evolution of structure can be simplified to a pure gasdynamical problem. It follows a box moving out at a constant speed, containing a representative portion of the structure generated by a radiatively driven model. The method allows us to follow structure out to more than a thousand stellar radii, at relatively low computational cost. We have used a less artificial line-strength cut-off than previous models. We also make a first attempt at improving the treatment of the energy balance in the wind.