Problem

The seat cushion is too soft, making the rear end sag too deeply.

Remove the seat cushions (three screws at the rear of the long one, two on the short one).
Cut a camping mattress and press it into place under the metal clamp that goes around the seat. If necessary, use two layers.

Cut two strips, about 10 and 5 cm with beveled edges and press into place (arrow) to get better side support.