I haven't seen anyone do that in many years but you can use material that is fastened in the back with elastic bands.
Is there something wrong with the back of the toilet?
Do you mean cover the cistern with something like Contact or do you mean enclosing the cistern? Or do you really mean the back of the cistern??
Hello Sarah , I'm assuming you mean the toilet wall? You can put some lovely wallpaper on it, or hang a gorgeous print or mirror.