What you need your counter ia radiation pressure gradients that would deform the shield. I’m assuming the craft rotates so that the centrifugal force keeps the shield flat and extended, so the craft itself would be a gyroscope on one axis. For average radiation pressure, the shield would need to be slightly closer to the Sun than the L1 point so that the pressure and the gravitational pull cancel each other out - you’ll still need at least two engines, one facing the Sun and other facing the Earth that would do orbit height adjustments. You may be able to get away with one if you shine lasers on the shield when you need to push it sunward.