  We model long-term variations in the scintillation of binary pulsar PSR
J1603$-$7202, observed by the 64 m Parkes radio telescope (Murriyang) between
2004 and 2016. We find that the time variation in the scintillation arc
curvature is well-modelled by scattering from an anisotropic thin screen of
plasma between the Earth and the pulsar. Using our scintillation model, we
measure the inclination angle and longitude of ascending node of the orbit,
yielding a significant improvement over the constraints from pulsar timing.
From our measurement of the inclination angle, we place a lower bound on the
mass of J1603$-$7202's companion of $\gtrsim 0.5\,\text{M}_\odot$ assuming a
pulsar mass of $\gtrsim1.2\,\text{M}_\odot$. We find that the scintillation
arcs are most pronounced when the electron column density along the line of
sight is increased, and that arcs are present during a known extreme scattering
event. We measure the distance to the interstellar plasma and its velocity, and
we discuss some structures seen in individual scintillation arcs within the
context of our model.
