For collage quilts I have used: Elmers washable school glue which then required stitching all elements in place, wonder under and misty fuse. Both wonder under and mistifuse also require stitching all elements in place, but I find it less messy to deal with fusibless thsn the glue and you don’t need to wash the quilt to remove the fusibles like you od with glue. Both wonderunder and mistyfuse are light and drapeable. Wonderunder has a release paper while mistfuse does not.
My process uses fusible just to tack the pieces in place long enough to get them to my machine. Some folks place a laer of tullee over the whole poece to help keep everything in place and then stitch. If you don’t mind some stiffness you could use an acid free glue stick that doesn’t wash out.
Rob