You have identified some of the problems with using templates with rotary cutters. Templates used to used with scissors in piecing. If the templates are all straight edge cuts, you need to find rigid materials to make them. Some people have found that places like a hardware store will cut shapes from acrylic for you. I have not had any luck in my area finding anyone to do that. You can use cereal box cardboard to make templates but you have to watch you don’t trim the edge while rotary cutting along your ruler. You can use a pencil to trace the shape on the wrong side of your fabric following the template but that is time consuming to then rotary cut on the lines. Maybe someone else has more suggestions for you.