Keep in mind that by the time you add your flannel, the roller blind may not roll up too easily ... or the old fashioned slide screen as it retracts into a tube.
When I was learning about design walls, I found a youtube video of one for travellers .... piece of fabric with rod pockets at top and bottom. Insert slats of wood. Sling over door, so the top one holds it over the door, and the bottom one just helps hold it down in place.
Put your stuff on it and then when you want it to disappear, it rolls up with all in place.
BTW ... instead of flannelette, I use polar fleece. Wider than flannelette plus everything sticks well to it.
If you have a wall large enough, you can put a piece of flannelette or flannel backed tablecloth on it with painters tape, then take down when not wanting to use. That's how I started when I first tried a design wall ... then mine grew into a permanent one! LOVE IT!!!!!