Absorbing wall along the beam propagation direction?

[niba125]

Hello I'm trying to implement Silicon Pore Optics, which can be modelled as an array of pores.
The first task is to define a pore, which can be seen as a square tube (square or rectangular section), where one face is a reflective surface (BentFlatMirror for instance) and the 3 others are not reflective (only absorbing).
Could you please advise on the implementation of the absorbing walls?

Many thanks!