A design wall?
Well, you could use anything from a piece of flannel or batting taped to the wall to a series of permanent insulation boards glued to the wall.
It really depends on how much room you have, and if you can leave it up semi-permanently or if you have to take it down occasionally (like, if it's in your living room).
Some people use a shade that can be rolled up.
It doesn't have to be an expensive solution.
I have insulation boards, but they're not attached to the wall so I could move them even though I don't have to.
What's your situation - do you have a wall or closet doors?
Are you looking for a permanent or temporary solution?