MTS

I use insulation boards.
They're around $12 at Home Depot or Lowes.
Size is 48"x96", but they can easily be cut to size if you need them a bit shorter or less wide.
Or if you need to stash them someplace unseen if you can't leave them out permanently.
They're covered on both sides.
I don't use ones that are just exposed styrofoam - too messy.

They come in a couple of thicknesses -I think 1/2" and 3/4 (or 7/8").
I use the 7/8" because they're sturdier just leaning against the wall. The thinner width doesn't necessary sag, but doesn't remain as ramrod straight.

I cover them with batting - you can use spray or staples or tape or thumbtacks to attach to the boards.

Because of the thickness, you can also use pins - just jab them in.
Which is why a harder surface would NOT work for me.
It limits what I can put up easily, and makes working with them a pain.

I have three boards lined up next to each other - I haven't bothered hanging them because I can't make up my mind about the final layout of other items in the room (hey, it's only been 3 years, what's the rush?).