I think I would get a pull down blind for the window by my sewing table and attach some flannel to it for a design wall. That way it you would be able to roll it up when you didn't need it. There's also a video link on here somewhere about laying out your project on a table and rolling it onto boards. Then you roll you a little at a time and baste, roll out a little more and baste. You might try purchasing some boards of different lengths and some folding tables that you could set up when you needed and take down when not in use. You have a lovely studio.
p.s. You can also use one of those cardboard folding cutting mats (used for dress patterns), a couple of cheap curtain rods, some flannel and some large bulldog clips. Attach the flannel to the board, then use the clips to attach it to the curtain rods. The rods keep it from folding up. When not in use, detach the clips, fold and store it away.