A method for modelling flows in a fractured medium crossed by a network of fluid conducting objects of definite geometry but not homogenizable on the scale of each grid cell of the model which has an application in the evaluation of hydrocarbon reservoirs is disclosed. The locking pin includes a head formed on one end and having an inner flat peripheral surface for solidly engaging with the ears, and includes an opening formed in a peripheral wall which may be expanded radially outward to engage with the ears, for solidly securing the locking pin to the ears. The closed end and sides of the cap snap into a cup-shaped extension positioned on an underside of the cover such that when the cover is closed over the PDB an open end of the cap receives the fastener.