Two towers on either side of the landing pad (say North and South), with two parallel main wires connecting the towers, one running on each side of the pad (say East and West). On each tower is a rail running East-West, and each main wire attaches to the rail through a slider that can run along the rail, allowing the East-West distance between the parallel main wires to be adjusted.
Connecting the two main wires are two minor wires (parallel to each other and perpendicular to the major wires) that attach with pulleys. The length of these minor wires must adjust with the distance between the main wires, and the distance between the parallel minor wires can furthermore be changed with the pulley system.
Together the four wires form a rectangle that can be expanded and contracted in both horizontal dimensions. They start out sufficiently wide apart that they are larger than Superheavy's horizontal precision. Superheavy descends down through the rectangle. After the engines pass through the height of the wires, the rectangle contracts until it is significantly smaller than the convex hull of the grid fins, but still larger than the rocket fuselage.
(Guy wires connect the tops of the tower to the ground to counteract horizontal forces on the tower as the load of Superheavy is applied.)