I haven't read everyone’s ideas but my thought is to make your quilt curtain and get a shower tension rod and line the quilt with a shower curtain and hang them together using a liner of fabric or a sheet for the backing with the plastic between Q. and backing. You can make tie backs to hold your curtain when you need it open. Hope this makes sense to you. It’s hard to get a clear idea of your problem from the picture, but maybe this will give you some idea Good luck
Carol