I use batting - when it gets too many strings I just use it up in a dark quilt! I have a row of nails that hang the batting from with binder clips, and then I tape the sides and bottom down with painter's tape. All very temporary and can be rolled up and stashed away (with work-in-progress still attached) if need be.
It works great until my big lumbering lummox of a dog comes by and wipes out half my blocks with her tail. Now I pin anything that's about waist-high or lower, LOL.