Depending on how long you'll be sitting in one place, a simple set of supports under the large topper will eliminate that problem. Some folks stuff beach ball under it, others buy special "pillows" made for the purpose. Since I sit on a site all summer, I made a set of supports from 1" PVC pipe that comes apart for storage. It's pretty easy to get it in and out of place, too.
Take a measurement of how high you want to raise the topper before putting the supports together. This gizmo actually sits close to the side wall of the rig once in place, allowing the topper to drain. The pieces for the supports are glued together, the long sections of pipe are not glued into the supports.
The three smaller toppers on my ODS side are tight and drain without a support under them.