I often slid forward in my seat, forcing me to shimmy back into position. This was especially the case after hitting a few bumps. I tried adjusting my backrest back further, but it continued to happen.
Mickey, if you ever doubt that your YouTube videos are worth the effort, let me assure you they are because that is where I discovered the solution -
The Terra Trike Wide Ultra Comfort Seat Wedge was the solution. It was the perfect width but was a little too tall for my need, which was not a problem; I just trimmed the thick end and sanded the edge to give it a custom fit.
Mickey, thank you for pointing the way to the right solution!