I just had something I framed a few years ago come back to haunt me. It's a Civil War document that is in tattered condition. I framed it floating on a dark background using mylar. The problem is, it's floating too much and has been slipping and sliding around inside the mylar so it is no longer centered. Is there a way to keep make it stay put? I left a bit of space between the piece and the mat, would cutting a mat with a smaller window opening help?