I like a towel or terry cloth cover. It holds or grips the block or fabric in place when I press , so I get less distortion. I use binder clips to hold in place and toss in the wash ever few weeks.

I covered a piece of 2' x 4' plywood and stapled my cover on it. Eventually got grungy - now I have a long piece of freezer paper ironed on. When it gets dirty - just rip it off and replace. Super easy.