Systems (100) and methods (600, 1800) for making a marker. The methods comprise: obtaining a resonator material which has been annealed under a tensile force selected to provide a maximum resonant amplitude at a bias field; providing by a bias material of the marker an operating bias field with a value less than a value of the bias field; forming a first housing portion from a flexible material so as to have a planar shape; and forming a second housing portion from the flexible material so as to comprise a cavity in which the resonator and bias materials can be housed when the second housing portion is coupled to the first housing portion. The cavity is defined by two opposing short sidewalls, two opposing elongate sidewalls (OESW) and a bottom sidewall. Each of OESW is stiffened by forming a plurality of first stiffener edge features along an exterior surface thereof.