Your tub chair is very nice and looks comfortable. It is not difficult to make a cover. make patterns of each section, I use muslin or old sheets, then add seam lines. Add any ease you want and be sure to make a pocket around the seat so it will give as you sit down. add elastic or ties to the bottom edge so it will be neat underneath. You can make it snug or loose and enjoy. I wish you much joy of your endeavor.