Exactly what I was looking for!  The cushions on the old sofa at my lakefront cottage are showing signs of wear and I didn't want to replace the whole thing.  Given that we often come inside in damp swimsuits these should be perfect covers.  I did some internet searches with no results and then had my "light bulb moment" - gee, I'll bet Wayfair might have what I'm looking for.  Sure enough, I found these right away.  Problem solved!