Remove the cushions from the sofa. Measure the cushion platform width from underneath the right arm to underneath the left arm. Measure the platform from the back cushion to 3 inches before the front edge.
Cut a piece of plywood the same size as you measured the cushion platform. Sand the edges to remove any slivers or rough spots that might snag the couch fabric.
Place the plywood on the cushion platform. Push it down so it's snug against the arms and fits flush against the surface.
Put the cushions back on the sofa. Your seat should now be firmer, allowing you to sit higher.