Hi Graham and welcome aboard!
I'm not aware of any commercially available cover as you describe. My wife made me a cover for my Tyros 5, out of black cloth. The goal was to keep the dust off. Being a baby boomer like me, her mother taught her how to sew. I'm not sure that talent exists anymore but it's worth looking into. Perhaps a local upholstery shop would make one for you at a decent price - assuming those guys still exist. Perhaps a seamstress?